xfs_scrub_all: fix argument passing when invoking xfs_scrub manually

xfs_scrub_all: fix argument passing when invoking xfs_scrub manually

Currently, xfs_scrub_all will try to invoke xfs_scrub with argv[1] being
"-n -x".  This of course is recognized by C getopt as a weird looking
string, not two individual arguments, and causes the child process to
exit with complaints about CLI usage.

What we really want is to split the string into a proper array and then
add them to the xfs_scrub command line.  The code here isn't strictly
correct, but as @scrub_args@ is controlled by us in the Makefile, it'll
do for now.

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