Fix O_APPEND for Linux 3.15 and older kernels

When using a Linux kernel which predates the iov_iter interface the
O_APPEND flag should be applied in zpl_aio_write() via the call to
generic_write_checks().  The updated pos variable  was incorrectly
ignored resulting in the current offset being used.

This issue should only realistically impact the RHEL/CentOS 7.x
kernels which are based on Linux 3.10.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13370 
Closes #13377

module/os/linux/zfs/zpl_file.c

@@ -417,6 +417,8 @@ zpl_aio_write(struct kiocb *kiocb, const struct iovec *iov,
 	if (ret)
 		return (ret);
 
+	kiocb->ki_pos = pos;
+
 	zfs_uio_t uio;
 	zfs_uio_iovec_init(&uio, iov, nr_segs, kiocb->ki_pos, UIO_USERSPACE,
 	    count, 0);