f2fs: fix not to drop mount options when retrying fill_super

If wrong mount option was requested, f2fs tries to fill_super again.
But, during the next trial, f2fs has no valid mount options, since
parse_options deleted all the separators in the original string.

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