fs: simplify vfs_get_super

Remove the pointless keying argument and associated enum and pass the
fill_super callback and a "bool reconf" instead.  Also mark the function
static given that there are no users outside of super.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>