ext4: return ENOMEM if sb_getblk() fails

The only reason for sb_getblk() failing is if it can't allocate the
buffer_head.  So ENOMEM is more appropriate than EIO.  In addition,
make sure that the file system is marked as being inconsistent if
sb_getblk() fails.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org