ext4: don't kfree uninitialized s_group_info members

We can call kfree on uninitialized members of the s_group_info array
on an the error path.  We can avoid this by kzalloc'ing the array.

This doesn't entirely solve the oops on mount if we fail down this
path; failed_mount4: frees the sbi, for one, which gets referenced
later in the failed mount paths - I haven't worked that out yet.

https://bugzilla.kernel.org/show_bug.cgi?id=30872

Reported-by: Eugene A. Shatokhin <dame_eugene@mail.ru>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>