rpc.mountd: make exportent->e_hostname a dynamically-allocated string

rpc.mountd: make exportent->e_hostname a dynamically-allocated string

This makes the e_hostname field of the exportent into a pointer to a
dynamically allocated string. This is necessary since this is field is
often filled out from the m_hostname. This too adds a few
micro-optimizations as we can avoid copying the string in some places
and simply pass a pointer to the original string instead.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.de>