xfs: make xfs_iroot_realloc take the new numrecs instead of deltas

xfs: make xfs_iroot_realloc take the new numrecs instead of deltas

Change the calling signature of xfs_iroot_realloc to take the ifork and
the new number of records in the btree block, not a diff against the
current number.  This will make the callsites easier to understand.

Note that this function is misnamed because it is very specific to the
single type of inode-rooted btree supported.  This will be addressed in
a subsequent patch.

Return the new btree root to reduce the amount of code clutter.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>