xfs_io: extract control number parsing routines

xfs_io: extract control number parsing routines

Break out the parts of parse_args that extract control numbers from the
CLI arguments, so that the function isn't as long.  This isn't all that
exciting now, but the scrub vectorization speedups will introduce a new
ioctl.  For the new command that comes with that, we'll want the control
number parsing helpers.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>