cache: decrease ncnegfactor to 5

The current mechanism is bogus in several ways:
- the limit is a percentage of total entries added, which means negative
entries get evicted all the time even if there are plenty of resources
- evicting code is almost not concurrent, which makes it unable to
remove entries fast enough when doing something as simple as -j 104
buildworld
- there is no support for performing mass removal if necessary

Vast majority of negative entries never get any hits. Only evicting
them when the filesystem demands it results in a significant growth of
the namecache with almost no improvement in the hit ratio.

Sample result about afer 90 minutes of poudriere -j 104:

           current    no evict   % of the original
numneg     219737     2013157    916
numneghits 266711906  263544562  98 [1]

[1] this may look funny but there is a certain dose of variation to the
build

The number was chosen as something which mostly eliminates spurious
evictions during lighter workloads but still keeps the total at bay.

Sponsored by:	The FreeBSD Foundation

sys/kern/vfs_cache.c

@@ -200,7 +200,7 @@ static __read_mostly LIST_HEAD(nchashhead, namecache) *nchashtbl;/* Hash Table *
 static u_long __read_mostly	nchash;			/* size of hash table */
 SYSCTL_ULONG(_debug, OID_AUTO, nchash, CTLFLAG_RD, &nchash, 0,
     "Size of namecache hash table");
-static u_long __read_mostly	ncnegfactor = 12; /* ratio of negative entries */
+static u_long __read_mostly	ncnegfactor = 5; /* ratio of negative entries */
 SYSCTL_ULONG(_vfs, OID_AUTO, ncnegfactor, CTLFLAG_RW, &ncnegfactor, 0,
     "Ratio of negative namecache entries");
 static u_long __exclusive_cache_line	numneg;	/* number of negative entries allocated */