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