fs: don't compile in drop_caches.c when CONFIG_SYSCTL=n

drop_caches.c provides code only invokable via sysctl, so don't compile it
in when CONFIG_SYSCTL=n.
Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/Makefile

@@ -10,7 +10,7 @@ obj-y :=	open.o read_write.o file_table.o super.o \
 		ioctl.o readdir.o select.o fifo.o dcache.o inode.o \
 		attr.o bad_inode.o file.o filesystems.o namespace.o \
 		seq_file.o xattr.o libfs.o fs-writeback.o \
-		pnode.o drop_caches.o splice.o sync.o utimes.o \
+		pnode.o splice.o sync.o utimes.o \
 		stack.o fs_struct.o statfs.o
 
 ifeq ($(CONFIG_BLOCK),y)
@@ -49,6 +49,7 @@ obj-$(CONFIG_FS_POSIX_ACL)	+= posix_acl.o xattr_acl.o
 obj-$(CONFIG_NFS_COMMON)	+= nfs_common/
 obj-$(CONFIG_GENERIC_ACL)	+= generic_acl.o
 obj-$(CONFIG_COREDUMP)		+= coredump.o
+obj-$(CONFIG_SYSCTL)		+= drop_caches.o
 
 obj-$(CONFIG_FHANDLE)		+= fhandle.o
 

include/linux/mm.h

@@ -1731,8 +1731,12 @@ int in_gate_area_no_mm(unsigned long addr);
 #define in_gate_area(mm, addr) ({(void)mm; in_gate_area_no_mm(addr);})
 #endif	/* __HAVE_ARCH_GATE_AREA */
 
+#ifdef CONFIG_SYSCTL
+extern int sysctl_drop_caches;
 int drop_caches_sysctl_handler(struct ctl_table *, int,
 					void __user *, size_t *, loff_t *);
+#endif
+
 unsigned long shrink_slab(struct shrink_control *shrink,
 			  unsigned long nr_pages_scanned,
 			  unsigned long lru_pages);

kernel/sysctl.c

@@ -106,7 +106,6 @@ extern unsigned int core_pipe_limit;
 #endif
 extern int pid_max;
 extern int pid_max_min, pid_max_max;
-extern int sysctl_drop_caches;
 extern int percpu_pagelist_fraction;
 extern int compat_log;
 extern int latencytop_enabled;