f2fs: use kfree() instead of kvfree() to free some memory

options in f2fs_fill_super is alloc by kstrdup:
	options = kstrdup((const char *)data, GFP_KERNEL)
sit_bitmap[_mir], nat_bitmap[_mir] are alloc by kmemdup:
	sit_i->sit_bitmap = kmemdup(src_bitmap, sit_bitmap_size, GFP_KERNEL);
	sit_i->sit_bitmap_mir = kmemdup(src_bitmap,
					sit_bitmap_size, GFP_KERNEL);
	nm_i->nat_bitmap = kmemdup(version_bitmap, nm_i->bitmap_size,
					GFP_KERNEL);
	nm_i->nat_bitmap_mir = kmemdup(version_bitmap, nm_i->bitmap_size,
					GFP_KERNEL);
write_io is alloc by f2fs_kmalloc:
	sbi->write_io[i] = f2fs_kmalloc(sbi,
			array_size(n, sizeof(struct f2fs_bio_info))

Use kfree is more efficient.

Signed-off-by: Jiazi Li <jqqlijiazi@gmail.com>
Signed-off-by: peixuan.qiu <peixuan.qiu@transsion.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>