quota: don't call sync_fs() from vfs_quota_off() when there's no quota turn off

Sometimes, vfs_quota_off() is called on a partially set up super block (for
example when fill_super() fails for some reason).  In such cases we cannot
call ->sync_fs() because it can Oops because of not properly filled in super
block.  So in case we find there's not quota to turn off, we just skip
everything and return which fixes the above problem.

[akpm@linux-foundation.org: fxi tpyo]
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>