target/file: merge fd_do_readv() and fd_do_writev()

Those two functions are almost identical so merge them. Noticed this
while fixing the highmem in both cases.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc drivers/target/target_core_file.c
Simple merge