nfsd: add more granular locking to forget_openowners fault injector

...instead of relying on the client_mutex.

Also, fix up the printk output that is generated when the file is read.
It currently says that it's reporting the number of open files, but
it's actually reporting the number of openowners.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>