new helper: dump_emit()

dump_write() analog, takes core_dump_params instead of file,
keeps track of the amount written in cprm->written and checks for
cprm->limit.  Start using it in binfmt_elf.c...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/binfmt_elf.c
Simple merge

diff --cc fs/coredump.c
Simple merge

diff --cc include/linux/binfmts.h
Simple merge

diff --cc include/linux/coredump.h
Simple merge