Fix man page installation path

RPM versions 4.8 and 4.9 differ in the definition of macro %_mandir:

$ rpm --version ; rpm --showrc | grep ^-14:._mandir
RPM version 4.9.0
-14: _mandir %{_prefix}/share/man

$ rpm --version ; rpm --showrc | grep ^-14:._mandir
RPM version 4.8.0
-14: _mandir /usr/share/man

zfs.spec.in defines %_prefix as /, so man pages end up getting
installed in /share/man on RPM 4.9 systems. To fix this, define
%_mandir relative to %_datadir in the spec file.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #353