xfs: rename xfs_bmap_add_free to xfs_free_extent_later

xfs: rename xfs_bmap_add_free to xfs_free_extent_later

Source kernel commit: c201d9ca5392b20f04882848a071025b0e194c17

xfs_bmap_add_free isn't a block mapping function; it schedules deferred
freeing operations for a later point in a compound transaction chain.
While it's primarily used by bunmapi, its use has expanded beyond that.
Move it to xfs_alloc.c and rename the function since it's now general
freeing functionality.  Bring the slab cache bits in line with the
way we handle the other intent items.

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