Fix the pNFS server for a case where mirror level equals number of DSs.

If a pNFS service was set up where the number of DSs equals the mirror level
and then a DS was disabled, the service would create files with duplicate
entries for the same DS. This bug occurred because I didn't realize that
TAILQ_FOREACH_FROM() would start at the beginning of the list when the
inital value of the variable was NULL.
This patch also changes the pNFS server DS file creation code so that it
creates entrie(s) with 0.0.0.0 IP address when it cannot create mirror level
files due to lack of DSs.
The patch only affects the pNFS service and only when it was created with
a number of DSs equal to the mirror level and mirroring is enabled.