f2fs: introduce fs_context_operation structure

The handle_mount_opt() helper is used to parse mount parameters,
and so we can rename this function to f2fs_parse_param() and set
it as .param_param in fs_context_operations.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
[sandeen: forward port]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>