virtiofs: simplify sb setup

Currently when acquiring an sb for virtiofs fuse_mount_get() is being
called from virtio_fs_set_super() if a new sb is being filled and
fuse_mount_put() is called unconditionally after sget_fc() returns.

The exact same result can be obtained by checking whether
fs_contex->s_fs_info was set to NULL (ref trasferred to sb->s_fs_info) and
only calling fuse_mount_put() if the ref wasn't transferred (error or
matching sb found).

This allows getting rid of virtio_fs_set_super() and fuse_mount_get().

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>