vfs: Create fs_context-aware mount_bdev() replacement

Create a function, vfs_get_block_super(), that is fs_context-aware and a
replacement for mount_bdev().  It caches the block device pointer and file
open mode in the fs_context struct so that this information can be passed
into sget_fc()'s test and set functions.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: linux-block@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>