kill the 4th argument of __generic_file_aio_write()

It's always equal to &iocb->ki_pos, where iocb is the value of the 1st
argument.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/block_dev.c
Simple merge

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

diff --cc fs/udf/file.c
Simple merge

diff --cc include/linux/fs.h
Simple merge

diff --cc mm/filemap.c
Simple merge