Btrfs-progs: fix magic return value in cmds-scrub.c

Btrfs-progs: fix magic return value in cmds-scrub.c

There will be four kinds of return value for command "scrub start":

0: scrub dosen't find errors and return success.
1: usage or syntax errors.
3: scrub finds errors and correct all of them.
4: scrub finds errors and some of them are not correctable.

Three kinds of return values for scrub cancel/resume:

0: cancel successfully.
1: usage or syntax errors.
2: cancel a not started or finished scrub.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>