exfat: avoid incorrectly releasing for root inode

In d_make_root, when we fail to allocate dentry for root inode,
we will iput root inode and returned value is NULL in this function.

So we do not need to release this inode again at d_make_root's caller.

Signed-off-by: Chen Li <chenli@uniontech.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>