f2fs: pass sbi rather than sb to quota qf_name helpers

With the new mount api we will not have the superblock available during
option parsing. Prepare for this by passing *sbi rather than *sb.

For now, we are parsing after fill_super has been done, so sbi->sb will
exist. Under the new mount API this will require more care, but do the
simple change for now.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>