f2fs: fix condition in __allow_reserved_blocks()

If reserve_root mount option is not assigned, __allow_reserved_blocks()
will return false, it's not correct, fix it.

Fixes: 7e65be49 ("f2fs: add reserved blocks for root user")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>