make dump_emit() use vfs_write() instead of banging at ->f_op->write directly

... and deal with short writes properly - the output might be to pipe, after
all; as it is, e.g. no-MMU case of elf_fdpic coredump can write a whole lot
more than a page worth of data at one call.

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