fuse: get rid of fuse_mount refcount

Fuse mount now only ever has a refcount of one (before being freed) so the
count field is unnecessary.

Remove the refcounting and fold fuse_mount_put() into callers.  The only
caller of fuse_mount_put() where fm->fc was NULL is fuse_dentry_automount()
and here the fuse_conn_put() can simply be omitted.

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