proc: move sysrq-trigger out of fs/proc/

Move it into sysrq.c, along with the rest of the sysrq implementation.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>