check return status of all write/fwrite functions as required by glibc 2.4

check return status of all write/fwrite functions as required by glibc 2.4

From: Luca Berra <bluca@vodka.it>

glibc 2.4 is pedantic on ignoring return values from fprintf, fwrite and
write, so now we check the rval and actually do something with it.
in the Grow.c case i only print a warning, since i don't think we can do
anything in case we fail invalidating those superblocks (is should never
happen, but then...)

Signed-off-by: Neil Brown <neilb@suse.de>