ext4: fix wrong return value in ext4_read_inode_bitmap()

The only reason that sb_getblk() could fail is out of memory,
ext4 codes have returned -ENOMME for all other places except this
one, let's fix it here too.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>