afs: Re-enable freezing once a page fault is interrupted

If a task is killed during a page fault, it does not currently call
sb_end_pagefault(), which means that the filesystem cannot be frozen
at any time thereafter.  This may be reported by lockdep like this:

====================================
WARNING: fsstress/10757 still has locks held!
5.13.0-rc4-build4+ #91 Not tainted
------------------------------------
1 lock held by fsstress/10757:
 #0: ffff888104eac530
 (
sb_pagefaults

as filesystem freezing is modelled as a lock.

Fix this by removing all the direct returns from within the function,
and using 'ret' to indicate whether we were interrupted or successful.

Fixes: 1cf7a15 ("afs: Implement shared-writeable mmap")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/20210616154900.1958373-1-willy@infradead.org/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Matthew Wilcox (Oracle)

fs/afs/write.c

@@ -837,6 +837,7 @@ vm_fault_t afs_page_mkwrite(struct vm_fault *vmf)
 	struct inode *inode = file_inode(file);
 	struct afs_vnode *vnode = AFS_FS_I(inode);
 	unsigned long priv;
+	vm_fault_t ret = VM_FAULT_RETRY;
 
 	_enter("{{%llx:%llu}},{%lx}", vnode->fid.vid, vnode->fid.vnode, page->index);
 
@@ -848,22 +849,22 @@ vm_fault_t afs_page_mkwrite(struct vm_fault *vmf)
 #ifdef CONFIG_AFS_FSCACHE
 	if (PageFsCache(page) &&
 	    wait_on_page_fscache_killable(page) < 0)
-		return VM_FAULT_RETRY;
+		goto out;
 #endif
 
 	if (wait_on_page_writeback_killable(page))
-		return VM_FAULT_RETRY;
+		goto out;
 
 	if (lock_page_killable(page) < 0)
-		return VM_FAULT_RETRY;
+		goto out;
 
 	/* We mustn't change page->private until writeback is complete as that
 	 * details the portion of the page we need to write back and we might
 	 * need to redirty the page if there's a problem.
 	 */
 	if (wait_on_page_writeback_killable(page) < 0) {
 		unlock_page(page);
-		return VM_FAULT_RETRY;
+		goto out;
 	}
 
 	priv = afs_page_dirty(page, 0, thp_size(page));
@@ -877,8 +878,10 @@ vm_fault_t afs_page_mkwrite(struct vm_fault *vmf)
 	}
 	file_update_time(file);
 
+	ret = VM_FAULT_LOCKED;
+out:
 	sb_end_pagefault(inode->i_sb);
-	return VM_FAULT_LOCKED;
+	return ret;
 }
 
 /*