NSM: Remove BUG_ON() in nsm_monitor()

Clean up: Remove the BUG_ON() invocation in nsm_monitor().  It's not
likely that nsm_monitor() is ever called with a NULL host pointer, and
the code will die anyway if host is NULL.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>