OpenZFS 8603 - rename zilog's "zl_writer_lock" to "zl_issuer_lock"

This is a purely cosmetic change. The zilog's "zl_writer_lock" field is
being renamed to "zl_issuer_lock" to try and make the code easier to
understand; no other changes are made.

Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: C Fraire <cfraire@me.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/8603
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/2daf06546b
Closes #6927