ext4: plug a buffer_head leak in an error path of ext4_iget()

One of the invalid error paths in ext4_iget() forgot to brelse() the
inode buffer head.  Fix it by adding a brelse() in the common error
return path, which also simplifies function.

Thanks to Andi Kleen <ak@linux.intel.com> reporting the problem.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>