Honor all options of "zfs mount -o"

[asomers]

Summary

Honor all options of "zfs mount -o"

The existing code in zmount incorrectly parses the comma-delimited option
string. The result is that nmount only honors the last option. AFAICT the
parsing has been broken ever since ZFS's initial import in change 168404.

PR: 222078

[avg]

LGTM.

head/cddl/compat/opensolaris/misc/zmount.c

SUBDIR= \
         ${_stg} \
         stge \
         ${_streams} \
         ${_svr4} \
         ${_sym} \
         ${_syscons} \
         sysvipc \
         ${_ti} \
-        tcp/fastpath \
+        ${_tcp_fastpath} \
         tests/framework \
         tests/callout_test \
         tl \
         tmpfs \
         ${_toecore} \
         ${_tpm} \
         trm \
         ${_twa} \

 _random_fortuna=random_fortuna
 _random_yarrow= random_yarrow
 _random_other=  random_other
 .endif
 .endif
 
 .if ${MK_CUSE} != "no" || defined(ALL_MODULES)
 SUBDIR+=        cuse
+.endif
+
+.if ${MK_EXTRA_TCP_STACKS} != "no" || defined(ALL_MODULES)
+_tcp_fastpath=  tcp/fastpath
 .endif
 
 .if (${MK_INET_SUPPORT} != "no" || ${MK_INET6_SUPPORT} != "no") || \
         defined(ALL_MODULES)
 _carp=          carp
 _toecore=       toecore
 _if_enc=        if_enc
 _if_gif=        if_gif