ext4: convert ext4_bread() to use the ERR_PTR convention

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --cc fs/ext4/dir.c
Simple merge

diff --cc fs/ext4/ext4.h
Simple merge

diff --cc fs/ext4/inode.c
Simple merge

diff --cc fs/ext4/namei.c
Simple merge

diff --cc fs/ext4/super.c
Simple merge