ext4: add missing save_error_info() to ext4_error()

The ext4_error() function is missing a call to save_error_info().
Since this is the function which marks the file system as containing
an error, this oversight (which was introduced in 2.6.36) is quite
significant, and should be backported to older stable kernels with
high urgency.

Change-Id: I21c5b80ab5685f731367f919ff0ce4248d865092
Reported-by: Ken Sumrall <ksumrall@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: ksumrall@google.com
Cc: stable@kernel.org
Signed-off-by: Ken Sumrall <ksumrall@android.com>