Originally from Anton Altaparmakov..

This causes errors from get_block() in block_read_full_page() to be
detected and handled properly (by setting page error flag).  Without the
patch the page (or parts of the page) will contain random data on
get_block() failing without any form of error being signalled which can
be catastrophic for filesystems using block_read_full_page() for
accessing their metadata.  And for normal data it would mean the user
would see random data instead of what they expected.