During testing of exporting file systems in jails, I
noticed that the export structures on a mount
were not being free'd when the mount is dismounted.
This bug appears to have been in the system for a
very long time. It would have resulted in a slow memory
leak when exported file systems were dismounted.
Prior to r362158, freeing the structures during dismount
would not have been safe, since VFS_CHECKEXP() returned
a pointer into an export structure, which might still be used
by the NFS server for an in-progress RPC when the file system
is dismounted. r362158 fixed this, so it should now be safe
to free the structures in vfs_mount_destroy(), which is what
this patch does.