frv: restart_block.fn needs to be reset on sigreturn

Reset restart_block.fn on executing a sigreturn such that any currently
pending system call restarts will be forced to return -EINTR.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>