xfs: introduce xfs_mod_frextents

Add a new helper to modify the incore counter of free realtime
extents. This matches the helpers used for inode and data block
counters, and removes a significant users of the xfs_mod_incore_sb()
interface.

Based on a patch originally from Christoph Hellwig.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --cc fs/xfs/libxfs/xfs_bmap.c
Simple merge

diff --cc fs/xfs/xfs_mount.c
Simple merge

diff --cc fs/xfs/xfs_mount.h
Simple merge

diff --cc fs/xfs/xfs_trans.c
Simple merge