xfs: Introduce xfs_iext_max_nextents() helper

xfs: Introduce xfs_iext_max_nextents() helper

Source kernel commit: 9feb8f19665c8ba051c6a81aa7897149e7748e1e

xfs_iext_max_nextents() returns the maximum number of extents possible for one
of data, cow or attribute fork. This helper will be extended further in a
future commit when maximum extent counts associated with data/attribute forks
are increased.

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