ext4: Don't leave behind a half-created inode if ext4_mkdir() fails

If ext4_mkdir() fails to allocate the initial block for the directory,
don't leave behind a half-created directory inode with the link count
left at one.  This was caused by an inappropriate call to ext4_dec_count().

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>