Fix a mismerge in r260183 which prevents snapshot zvol devices being
removed and re-instate the fix in r242862.

Reported by:	Leon Dang <ldang nahannisys com>, smh
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272324 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f