xtensa: fix lock_mm_and_find_vma in case VMA not found

MMU version of lock_mm_and_find_vma releases the mm lock before
returning when VMA is not found. Do the same in noMMU version.
This fixes hang on an attempt to handle protection fault.

Fixes: d85a143b ("xtensa: fix NOMMU build with lock_mm_and_find_vma() conversion")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/nommu.c

@@ -637,8 +637,13 @@ EXPORT_SYMBOL(find_vma);
 struct vm_area_struct *lock_mm_and_find_vma(struct mm_struct *mm,
 			unsigned long addr, struct pt_regs *regs)
 {
+	struct vm_area_struct *vma;
+
 	mmap_read_lock(mm);
-	return vma_lookup(mm, addr);
+	vma = vma_lookup(mm, addr);
+	if (!vma)
+		mmap_read_unlock(mm);
+	return vma;
 }
 
 /*