f2fs: allow empty mount string for Opt_usr|grp|projjquota

f2fs: allow empty mount string for Opt_usr|grp|projjquota

The fsparam_string_empty() gives an error when mounting without string, since
its type is set to fsparam_flag in VFS. So, let's allow the flag as well.

This addresses xfstests/f2fs/015 and f2fs/021.

Fixes: d18535132523 ("f2fs: separate the options parsing and options checking")
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>