fs/exec.c: make do_coredump() void

No one cares do_coredump()'s return value, and also it seems that it
is also not necessary. So make it void.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: WANG Cong <wangcong@zeuux.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>