Don't assert random_adaptors_lock in random_adaptor_read_rate().

Reported by:	many
Pointy hat to:	delphij (for not testing the commit with WITNESS)
Approved by:	so


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274006 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f