fs/buffer.c: record blockdev write errors in super_block that it backs

When syncing out a block device (a'la __sync_blockdev), any error
encountered will only be recorded in the bd_inode's mapping.  When the
blockdev contains a filesystem however, we'd like to also record the
error in the super_block that's stored there.

Make mark_buffer_write_io_error also record the error in the
corresponding super_block when a writeback error occurs and the block
device contains a mounted superblock.

Since superblocks are RCU freed, hold the rcu_read_lock to ensure that
the superblock doesn't go away while we're marking it.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andres Freund <andres@anarazel.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Link: http://lkml.kernel.org/r/20200428135155.19223-3-jlayton@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>