MFC r323193:

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
Reviewed by: avg
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12232