ext4: split ext4_file_write into buffered IO and direct IO
ext4: split ext4_file_write into buffered IO and direct IO

ext4_file_dio_write is defined in order to split buffered IO and
direct IO in ext4.  This patch just refactor some stuff in write path.

CC: Tao Ma <a547db31280d14f095c760a23a7523d0b1e55fe6@tao.ma>
CC: Eric Sandeen <d55521028c14e0f378be5444e4ea388161c0d7e7@redhat.com>
CC: Robin Dong <e7669e51787303abfdbf53f8ede7d886db6c355f@gmail.com>
Signed-off-by: Zheng Liu <927570c33013ae711b16b2f69d88304bb3fcc969@taobao.com>
Signed-off-by: "Theodore Ts'o" <4ed386e0495d3e109932df055831d9ec2f824927@mit.edu>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas