xfstests: fix _mount_ops_filter() to support $TEST_DIR containing -o

xfstests: fix _mount_ops_filter() to support $TEST_DIR containing -o

If multiple mount options are given with multiple -o parameters,
_mount_ops_filter concatenates them by replacing all -o's with a comma
(except for the first -o).

If $TEST_DIR contains "-o" (e.g. /mnt/test-os/) you get errors like this:

   # ./check generic/002
     mount: mount point /mnt/test,s/ does not exist
     common/rc: retrying test device mount with external set
     mount: mount point /mnt/test,s/ does not exist
     common/rc: could not mount /dev/sdc3 on /mnt/test-os/

Fix it by only replacing -o's that are preceded by at least one space.

Signed-off-by: Koen De Wit <koen.de.wit@oracle.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>