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>

module/zfs/arc.c

@@ -6103,7 +6103,9 @@ top:
 			ARCSTAT_CONDSTAT(!(*arc_flags & ARC_FLAG_PREFETCH),
 			    demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data,
 			    metadata, misses);
-			zfs_racct_read(spa, size, 1, 0);
+			zfs_racct_read(spa, size, 1,
+			    (*arc_flags & ARC_FLAG_UNCACHED) ?
+			    DMU_UNCACHEDIO : 0);
 		}
 
 		/* Check if the spa even has l2 configured */