mountd: Fix updating the network/host(s) for an exports line

Mountd reloads the exports(5) file(s) when it receives a SIGHUP
and then compares the old and new exports, updating any ones that
have changed in the kernel.

Without this patch, mountd failed to recognize that a network/host(s)
had changed, if there was no other change to the exports line.
As such, the change of network/hosts(s) did not take effect until
the mountd daemon was (re)started.

This patch fixes the code so that it checks for changes in
the network/host(s) list for an exports line.

PR: 286260
MFC after: 3 days