xfs: factor out a xfs_update_prealloc_flags() helper

This logic is duplicated in xfs_file_fallocate and xfs_ioc_space, and
we'll need another copy of it for pNFS block support.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>