Fix support for LUN flat space addressing.

MFC after:	1 week

(cherry picked from commit be81b72040e8d5829a1a9f3453e9f89e3d985aa7)
Allocate buffer for READ BUFFER/WRITE BUFFER commands on demand.

These commands are rare, but consume additional 256KB RAM per LUN.

MFC after:	1 week

(cherry picked from commit 6a4642451ed02bdd3f5f5f5f0a1c2cc4d4dd79ab)
Reduce code duplication around Write Exclusive persistent reservation.

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

MFC after:	1 week

(cherry picked from commit 1a960f7ebea1c5855e9a85ab1bf24a542af0d052)
Remove comment obsoleted by r273730.

MFC after:	1 week

(cherry picked from commit 981411c14ee33b40717b9ca1151bd411b7add9b7)