fstests: add missing kernel git commit IDs to some tests

Three tests (btrfs/321, generic/364 and xfs/608) refer to kernel patches
that are now in Linus' git kernel tree, so update the tests to include
the commit IDs.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>