Reduce code duplication around Write Exclusive persistent reservation.

While there, allow some more commands to pass persistent reservation.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273730 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Remove comment obsoleted by r273730.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273731 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f