xfs_restore: support fallocate when reserving space for a file

Update the file creation helper to try fallocate when restoring a
filesystem before it tries RESVSP.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>