f2fs: return success if there is no work to do

Static analysis reports this problem
file.c:3206:2: warning: Undefined or garbage value returned to caller
        return err;
        ^~~~~~~~~~

err is only set if there is some work to do.  Because the loop returns
immediately on an error, if all the work was done, a 0 would be returned.
Instead of checking the unlikely case that there was no work to do,
change the return of err to 0.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>