nfsd4: reserve adequate space for LOCK op

As of  8c7424cf "nfsd4: don't try to encode conflicting owner if low
on space", we permit the server to process a LOCK operation even if
there might not be space to return the conflicting lockowner, because
we've made returning the conflicting lockowner optional.

However, the rpc server still wants to know the most we might possibly
return, so we need to take into account the possible conflicting
lockowner in the svc_reserve_space() call here.

Symptoms were log messages like "RPC request reserved 88 but used 108".

Fixes: 8c7424cf "nfsd4: don't try to encode conflicting owner if low on space"
Reported-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>