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=12370Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
cc: stable@kernel.org