The recently added feature of the pNFS server will set an fsid for the
MDS file system to define the file system a DS should store files for.
For a case where a DS handling all file systems has failed, it was possible
for the code to check for a mirror with a specified fs, even though
nfsdev_mdsisset was 0, possibly causing a false successful check for a mirror.
This patch adds a check for nfsdev_mdsisset != 0 to avoid this.
It only affects the pNFS server for a rare case. Found via code inspection.
bc2294b707