jbd2: Fix oops in jbd2_journal_init_inode() on corrupted fs

On 32-bit system with CONFIG_LBD getblk can fail because provided
block number is too big.  Add error checks so we fail gracefully if
getblk() returns NULL (which can also happen on memory allocation
failures).

Thanks to David Maciejak from Fortinet's FortiGuard Global Security
Research Team for reporting this bug.

http://bugzilla.kernel.org/show_bug.cgi?id=12370

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
cc: stable@kernel.org