f2fs: move error condition for mkdir at proper place

In function f2fs_mkdir, err is being initialized without even checking
if there was any error in new inode creation. So, instead check the
inode error and make use of error/return condition.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>