Remove GRUB restrictions

The GRUB restrictions are based around the pool's bootfs property.
Given the current situation where GRUB is not staying current with
OpenZFS pool features, having either a non-ZFS /boot or a separate
pool with limited features are pretty much the only long-term answers
for GRUB support.  Only the second case matters in this context.  For
the restrictions to be useful, the bootfs property would have to be set
on the boot pool, because that is where we need the restrictions, as
that is the pool that GRUB reads from. The documentation for bootfs
describes it as pointing to the root pool. That's also how it's used in
the initramfs. ZFS does not allow setting bootfs to point to a dataset
in another pool. (If it did, it'd be difficult-to-impossible to enforce
these restrictions cross-pool). Accordingly, bootfs is pretty much
useless for GRUB scenarios moving forward.

Even for users who have only one pool, the existing restrictions for
GRUB are incomplete. They don't prevent you from enabling the
unsupported checksums, for example. For that reason, I have ripped out
all the GRUB restrictions.

A little longer-term, I think extending the proposed features=portable
system to define a features=grub is a much more useful approach. The
user could set that on the boot pool at creation, and things would
Just Work.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8627

man/man5/zpool-features.5

@@ -340,9 +340,6 @@ can turn on the \fBedonr\fR checksum on any dataset using the
 and will return to being \fBenabled\fR once all filesystems that have
 ever had their checksum set to \fBedonr\fR are destroyed.
 
-The \fBedonr\fR feature is not supported by GRUB and must not be used on
-the pool if GRUB needs to access the pool (e.g. for /boot).
-
 FreeBSD does not support the \fBedonr\fR feature.
 .RE
 
@@ -827,9 +824,6 @@ can turn on the \fBsha512\fR checksum on any dataset using
 \fBactive\fR once a \fBchecksum\fR property has been set to \fBsha512\fR,
 and will return to being \fBenabled\fR once all filesystems that have
 ever had their checksum set to \fBsha512\fR are destroyed.
-
-The \fBsha512\fR feature is not supported by GRUB and must not be used on
-the pool if GRUB needs to access the pool (e.g. for /boot).
 .RE
 
 .sp
@@ -861,9 +855,6 @@ can turn on the \fBskein\fR checksum on any dataset using
 \fBactive\fR once a \fBchecksum\fR property has been set to \fBskein\fR,
 and will return to being \fBenabled\fR once all filesystems that have
 ever had their checksum set to \fBskein\fR are destroyed.
-
-The \fBskein\fR feature is not supported by GRUB and must not be used on
-the pool if GRUB needs to access the pool (e.g. for /boot).
 .RE
 
 .sp

man/man8/zfsprops.8

@@ -762,8 +762,6 @@ The
 and
 .Sy edonr
 checksum algorithms require enabling the appropriate features on the pool.
-These pool features are not supported by GRUB and must not be used on the
-pool if GRUB needs to access the pool (e.g. for /boot).
 FreeBSD does not support the
 .Sy edonr
 algorithm.