f2fs: clean up f2fs_map_blocks

f2fs_map_blocks():

if (blkaddr == NEW_ADDR || blkaddr == NULL_ADDR) {
	if (create) {
		...
	} else {
		...
		if (flag == F2FS_GET_BLOCK_FIEMAP &&
					blkaddr == NULL_ADDR) {
			...
		}
		if (flag != F2FS_GET_BLOCK_FIEMAP ||
					blkaddr != NEW_ADDR)
			goto sync_out;
	}

It means we can break the loop in cases of:
a) flag != F2FS_GET_BLOCK_FIEMAP or
b) flag == F2FS_GET_BLOCK_FIEMAP && blkaddr == NULL_ADDR

Condition b) is the same as previous one, so merge operations of them
for readability.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>