xfs: assert that we hold the ilock for extent map access

Make sure that xfs_bmapi_read has the ilock held in some way, and that
xfs_bmapi_write, xfs_bmapi_delay, xfs_bunmapi and xfs_iread_extents are
called with the ilock held exclusively.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>