ext4: fix potential NULL pointer dereference in ext4_fill_super()

ext4: fix potential NULL pointer dereference in ext4_fill_super()

By mistake we fail to return an error from ext4_fill_super() in case
that ext4_alloc_sbi() fails to allocate a new sbi. Instead we just set
the ret variable and allow the function to continue which will later
lead to a NULL pointer dereference. Fix it by returning -ENOMEM in the
case ext4_alloc_sbi() fails.

Fixes: cebe85d570cf ("ext4: switch to the new mount api")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Link: https://lore.kernel.org/r/20220119130209.40112-1-lczerner@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org