Wire O_DIRECT also to Uncached I/O (#17218)

Before Direct I/O was implemented, I've implemented lighter version
I called Uncached I/O.  It uses normal DMU/ARC data path with some
optimizations, but evicts data from caches as soon as possible and
reasonable.  Originally I wired it only to a primarycache property,
but now completing the integration all the way up to the VFS.

While Direct I/O has the lowest possible memory bandwidth usage,
it also has a significant number of limitations.  It require I/Os
to be page aligned, does not allow speculative prefetch, etc.  The
Uncached I/O does not have those limitations, but instead require
additional memory copy, though still one less than regular cached
I/O.  As such it should fill the gap in between.  Considering this
I've disabled annoying EINVAL errors on misaligned requests, adding
a tunable for those who wants to test their applications.

To pass the information between the layers I had to change a number
of APIs.  But as side effect upper layers can now control not only
the caching, but also speculative prefetch.  I haven't wired it to
VFS yet, since it require looking on some OS specifics.  But while
there I've implemented speculative prefetch of indirect blocks for
Direct I/O, controllable via all the same mechanisms.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Fixes #17027
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>

lib/libzpool/zfs_racct.c

@@ -27,13 +27,13 @@
 #include <sys/zfs_racct.h>
 
 void
-zfs_racct_read(spa_t *spa, uint64_t size, uint64_t iops, uint32_t flags)
+zfs_racct_read(spa_t *spa, uint64_t size, uint64_t iops, dmu_flags_t flags)
 {
 	(void) spa, (void) size, (void) iops, (void) flags;
 }
 
 void
-zfs_racct_write(spa_t *spa, uint64_t size, uint64_t iops, uint32_t flags)
+zfs_racct_write(spa_t *spa, uint64_t size, uint64_t iops, dmu_flags_t flags)
 {
 	(void) spa, (void) size, (void) iops, (void) flags;
 }