m68k: Add missing mmap_read_lock() to sys_cacheflush()

When the superuser flushes the entire cache, the mmap_read_lock() is not
taken, but mmap_read_unlock() is called.  Add the missing
mmap_read_lock() call.

Fixes: cd2567b6 ("m68k: call find_vma with the mmap_sem held in sys_cacheflush()")
Signed-off-by: NLiam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20210407200032.764445-1-Liam.Howlett@Oracle.comSigned-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/kernel/sys_m68k.c

@@ -388,6 +388,8 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
 		ret = -EPERM;
 		if (!capable(CAP_SYS_ADMIN))
 			goto out;
+
+		mmap_read_lock(current->mm);
 	} else {
 		struct vm_area_struct *vma;