MFC r305207: MFV r302659: 6931 lib/libzfs: cleanup gcc warnings

illumos/illumos-gate@88f61dee20b358671b1b643e9d1dbf220a1d69be
https://github.com/illumos/illumos-gate/commit/88f61dee20b358671b1b643e9d1dbf220a1d69be

https://www.illumos.org/issues/6931

need cleanup:
CERRWARN += -_gcc=-Wno-switch
CERRWARN += -_gcc=-Wno-parentheses
CERRWARN += -_gcc=-Wno-unused-function

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Igor Kozhukhov <ikozhukhov@gmail.com>