openzfs: fix gcc kernel module builds

• Suppress -Wredundant-decls. Ultimately this warning is harmless in any case, and it does not look like there is a simple way to avoid redundant declarations in this case without a lot of header pollution (e.g. having openzfs's shim param.h pulling in sys/kernel.h for hz).
• Suppress -Wnested-externs, which is useless anyway.

Unfortunately it was not sufficient just to modify OPENZFS_CFLAGS,
because the warning suppressions need to appear on the command line
after they are explicitly enabled by CWARNFLAGS from sys/conf/kern.mk,
but OPENZFS_CFLAGS get added before due to use of -I for the shims.

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27685