xfs: compute the maximum height of the rmap btree when reflink enabled

xfs: compute the maximum height of the rmap btree when reflink enabled

Source kernel commit: 9ec691205e7d4a11190519df6561a168ae6af3a4

Instead of assuming that the hardcoded XFS_BTREE_MAXLEVELS value is big
enough to handle the maximally tall rmap btree when all blocks are in
use and maximally shared, let's compute the maximum height assuming the
rmapbt consumes as many blocks as possible.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>