ext4: use atomic functions to set bh_state

Use the BUFFER_FNS functions (set_buffer_foo) to set buffer
head state atomically instead of nonatomic __set_bit().

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>