Add '-u' - nomount flag for zfs set

This commit adds '-u' flag for zfs set operation. With this flag,
mountpoint, sharenfs and sharesmb properties can be updated
without actually mounting or sharing the dataset.

Previously, if dataset was unmounted, and mountpoint property was
updated, dataset was not mounted after the update. This behavior
is changed in #15240. We mount the dataset whenever mountpoint
property is updated, regardless if it's mounted or not.

To provide the user with option to keep the dataset unmounted and
still update the mountpoint without mounting the dataset, '-u'
flag can be used.

If any of mountpoint, sharenfs or sharesmb properties are updated
with '-u' flag, the property is set to desired value but the
operation to (re/un)mount and/or (re/un)share the dataset is not
performed and dataset remains as it was before.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #15322

man/man7/zfsprops.7

@@ -1248,10 +1248,18 @@ Otherwise, they are automatically remounted in the new location if the property
 was previously
 .Sy legacy
 or
-.Sy none ,
-or if they were mounted before the property was changed.
+.Sy none .
 In addition, any shared file systems are unshared and shared in the new
 location.
+.Pp
+When the
+.Sy mountpoint
+property is set with
+.Nm zfs Cm set Fl u
+, the
+.Sy mountpoint
+property is updated but dataset is not mounted or unmounted and remains
+as it was before.
 .It Sy nbmand Ns = Ns Sy on Ns | Ns Sy off
 Controls whether the file system should be mounted with
 .Sy nbmand
@@ -1656,6 +1664,13 @@ by default.
 This means that any additional access control
 (disallow specific user specific access etc) must be done on the underlying file
 system.
+.Pp
+When the
+.Sy sharesmb
+property is updated with
+.Nm zfs Cm set Fl u
+, the property is set to desired value, but the operation to share, reshare
+or unshare the the dataset is not performed.
 .It Sy sharenfs Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Ar opts
 Controls whether the file system is shared via NFS, and what options are to be
 used.
@@ -1699,6 +1714,13 @@ or if they were shared before the property was changed.
 If the new property is
 .Sy off ,
 the file systems are unshared.
+.Pp
+When the
+.Sy sharenfs
+property is updated with
+.Nm zfs Cm set Fl u
+, the property is set to desired value, but the operation to share, reshare
+or unshare the the dataset is not performed.
 .It Sy logbias Ns = Ns Sy latency Ns | Ns Sy throughput
 Provide a hint to ZFS about handling of synchronous requests in this dataset.
 If