libshare: nfs: pass through ipv6 addresses in bracket notation

Recognize when the host part of a sharenfs attribute is an ipv6
Literal and pass that through without modification.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Felix Dörre <felix@dogcraft.de>
Closes: #11171
Closes #11939
Closes: #1894