generic_perform_write()/iomap_write_actor(): saner logics for short copy

mainline inclusion
from mainline-v5.13
commit bc1bb416
category: bugfix
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I5HAC1


CVE: NA

Intel-SIG: commit bc1bb416 generic_perform_write()/iomap_write_actor():
saner logics for short copy.
Backport for MCA recovery enhance and bug fix.

--------------------------------

if we run into a short copy and ->write_end() refuses to advance at all,
use the amount we'd managed to copy for the next iteration to handle.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Youquan Song <youquan.song@intel.com>