ipc/shm: fix crash if CONFIG_SHMEM is not set

Commit c01d5b300774 ("shmem: get_unmapped_area align huge page") makes
use of shm_get_unmapped_area() in shm_file_operations() unconditional to
CONFIG_MMU.

As Tony Battersby pointed this can lead NULL-pointer dereference on
machine with CONFIG_MMU=y and CONFIG_SHMEM=n.  In this case ipc/shm is
backed by ramfs which doesn't provide f_op->get_unmapped_area for
configurations with MMU.

The solution is to provide dummy f_op->get_unmapped_area for ramfs when
CONFIG_MMU=y, which just call current->mm->get_unmapped_area().

Fixes: c01d5b300774 ("shmem: get_unmapped_area align huge page")
Link: http://lkml.kernel.org/r/20160912102704.140442-1-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Tony Battersby <tonyb@cybernetics.com>
Tested-by: Tony Battersby <tonyb@cybernetics.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>	[4.7.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ramfs/file-mmu.c

@@ -27,9 +27,17 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/ramfs.h>
+#include <linux/sched.h>
 
 #include "internal.h"
 
+static unsigned long ramfs_mmu_get_unmapped_area(struct file *file,
+		unsigned long addr, unsigned long len, unsigned long pgoff,
+		unsigned long flags)
+{
+	return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
+}
+
 const struct file_operations ramfs_file_operations = {
 	.read_iter	= generic_file_read_iter,
 	.write_iter	= generic_file_write_iter,
@@ -38,6 +46,7 @@ const struct file_operations ramfs_file_operations = {
 	.splice_read	= generic_file_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.llseek		= generic_file_llseek,
+	.get_unmapped_area	= ramfs_mmu_get_unmapped_area,
 };
 
 const struct inode_operations ramfs_file_inode_operations = {