xfs: buffer items don't straddle pages anymore

Unmapped buffers don't exist anymore, so the page straddling
detection and slow path code can go away now.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>