xfs: uncached buffer reads need to return an error

With verification being done as an IO completion callback, different
errors can be returned from a read. Uncached reads only return a
buffer or NULL on failure, which means the verification error cannot
be returned to the caller.

Split the error handling for these reads into two - a failure to get
a buffer will still return NULL, but a read error will return a
referenced buffer with b_error set rather than NULL. The caller is
responsible for checking the error state of the buffer returned.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Phil White <pwhite@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_mount.c

@@ -658,6 +658,12 @@ reread:
 			xfs_warn(mp, "SB buffer read failed");
 		return EIO;
 	}
+	if (bp->b_error) {
+		error = bp->b_error;
+		if (loud)
+			xfs_warn(mp, "SB validate failed");
+		goto release_buf;
+	}
 
 	/*
 	 * Initialize the mount structure from the superblock.