Copy generic_file_aio_write() into ext4_file_write().  This is part of
a patch series which allows us to simplify ext4_file_write() and
ext4_file_dio_write(), by calling __generic_file_aio_write() directly.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: NJan Kara <jack@suse.cz>