ext4: return ENOMEM if sb_getblk() fails

ext4: return ENOMEM if sb_getblk() fails

commit 860d21e2c585f7ee8a4ecc06f474fdc33c9474f4 upstream.

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>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop change to inline.c
 - Call to ext4_ext_check() from ext4_ext_find_extent() is conditional]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>