fsx: add support for RWF_DONTCACHE

fsx: add support for RWF_DONTCACHE

Using RWF_DONTCACHE tells the kernel that any page cache instantiated
by this operation should get pruned once the operation completes. If
data is in cache prior to the operation it will remain there.

Add ops for testing both the read and write side of this. At startup,
kernel support for this feature is probed. If support isn't available,
uncached/dontcache IO is performed as regular buffered IO. If -Z is
used to turn on O_DIRECT, then uncached/dontcache IO isn't performed.
Defaults to on if available, and adds a -T parameter to turn it off.

See the kernel posting adding support:

https://lore.kernel.org/linux-fsdevel/20241220154831.1086649-1-axboe@kernel.dk/

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>