proc: switch /proc/loadavg to seq_file

and move it to fs/proc/loadavg.c while I'm at it.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>

fs/proc/Makefile

@@ -9,7 +9,7 @@ proc-$(CONFIG_MMU)	:= mmu.o task_mmu.o
 
 proc-y       += inode.o root.o base.o generic.o array.o \
 		proc_tty.o proc_misc.o
-
+proc-y	+= loadavg.o
 proc-$(CONFIG_PROC_SYSCTL)	+= proc_sysctl.o
 proc-$(CONFIG_NET)		+= proc_net.o
 proc-$(CONFIG_PROC_KCORE)	+= kcore.o