ext4: Fix null dereference in ext4_fill_super()

Fix failure paths in ext4_fill_super() that can lead to a null
dereference.  This was designated CVE-2015-8324.

Mostly extracted from commit 744692dc ("ext4: use
ext4_get_block_write in buffer write").

However there's one more incorrect goto to fix, removed upstream in
commit cf40db13 ("ext4: remove failed journal checksum check").

Reference: https://bugs.openvz.org/browse/OVZ-6541


Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Willy Tarreau <w@1wt.eu>