f2fs: add default mount options to remount

I use f2fs filesystem with /data partition on my Android phone
by the default mount options. When I remount /data in order to
adding discard option to run some benchmarks, I find the default
options such as background_gc, user_xattr and acl turned off.

So I introduce a function named default_options in super.c. It do
some default setting, and both mount and remount operations will
call this function to complete default setting.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>