cifs: report error instead of invalid when revalidating a dentry fails

Assuming
- //HOST/a is mounted on /mnt
- //HOST/b is mounted on /mnt/b

On a slow connection, running 'df' and killing it while it's
processing /mnt/b can make cifs_get_inode_info() returns -ERESTARTSYS.

This triggers the following chain of events:
=> the dentry revalidation fail
=> dentry is put and released
=> superblock associated with the dentry is put
=> /mnt/b is unmounted

This patch makes cifs_d_revalidate() return the error instead of 0
(invalid) when cifs_revalidate_dentry() fails, except for ENOENT (file
deleted) and ESTALE (file recreated).
Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Suggested-by: Shyam Prasad N <nspmangalore@gmail.com>
Reviewed-by: Shyam Prasad N <nspmangalore@gmail.com>
CC: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/dir.c

@@ -737,6 +737,7 @@ static int
 cifs_d_revalidate(struct dentry *direntry, unsigned int flags)
 {
 	struct inode *inode;
+	int rc;
 
 	if (flags & LOOKUP_RCU)
 		return -ECHILD;
@@ -746,8 +747,25 @@ cifs_d_revalidate(struct dentry *direntry, unsigned int flags)
 		if ((flags & LOOKUP_REVAL) && !CIFS_CACHE_READ(CIFS_I(inode)))
 			CIFS_I(inode)->time = 0; /* force reval */
 
-		if (cifs_revalidate_dentry(direntry))
-			return 0;
+		rc = cifs_revalidate_dentry(direntry);
+		if (rc) {
+			cifs_dbg(FYI, "cifs_revalidate_dentry failed with rc=%d", rc);
+			switch (rc) {
+			case -ENOENT:
+			case -ESTALE:
+				/*
+				 * Those errors mean the dentry is invalid
+				 * (file was deleted or recreated)
+				 */
+				return 0;
+			default:
+				/*
+				 * Otherwise some unexpected error happened
+				 * report it as-is to VFS layer
+				 */
+				return rc;
+			}
+		}
 		else {
 			/*
 			 * If the inode wasn't known to be a dfs entry when