ext4: move extra inode read to a new function

Currently, in ext4_iget we do a simple check to see whether
there does exist some information starting from the end
of i_extra_size. With inline data added, this procedure
is more complicated. So move it to a new function named
ext4_iget_extra_inode.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>