Fix the scan handling for 11b->11g upgrades in a world where, well,
it's not just 11b/11g.

The following was happening, and it's quite .. annoyingly grr-y.

* create vap, setup wpa_supplicant with no bgscanning, etc - there's
  no call to ieee80211_media_change, so vap->iv_des_mode is
  IEEE80211_MODE_AUTO;
* do ifconfig wlan0 scan - same thing, media_change doesn't get called,
  iv_des_mode stays as auto.
* But then, run wpa_cli and do 'scan' - it'll do a media change.
* if you're on 11ng, vap->iv_des_mode gets changed to IEEE80211_MODE_11NG
* Then makescanlist() is called.  There's a block of code that gets
  called if iv_des_mode != IEEE80211_MODE_AUTO, and it does this:

		if (vap->iv_des_mode != IEEE80211_MODE_11G ||
			    mode != IEEE80211_MODE_11B)
			continue;
		mode = IEEE80211_MODE_11G;	/* upgrade */

* .. now, iv_des_mode is not IEEE80211_MODE_11G, so it always runs
  'continue'
* .. and thus the scan list stays empty and no further channel
  scans occur. Ever.(1)

If you then disassociate and try associating to something, your
scan table has likely been purged / aged out and you'll never
see anything in the scan list.

(1) You need to do 'ifconfig wlan0 mode auto' or just destroy/re-create
    the VAP to get working wireless again.

Tested:

* iwn(4) - intel 5300 wifi; STA mode; using wpa_supplicant; bgscan
  enabled -and- wpa_supplicant scanning.

Thanks to:

* Everyone who kept poking me about this and wondering why the hell
  their wifi would eventually stop seeing scan lists.  Grr.
  I eventually snapped this evening and dug back into this code.
The VOP_LOOKUP() implementations for CREATE op do not put the name
into namecache, to avoid cache trashing when doing large operations.
E.g., tar archive extraction is not usually followed by access to many
of the files created.

Right now, each VOP_LOOKUP() implementation explicitely knowns about
this quirk and tests for both MAKEENTRY flag presence and op != CREATE
to make the call to cache_enter().  Centralize the handling of the
quirk into VFS, by deciding to cache only by MAKEENTRY flag in VOP.
VFS now sets NOCACHE flag for CREATE namei() calls.

Note that the change in semantic is backward-compatible and could be
merged to the stable branch, and is compatible with non-changed
third-party filesystems which correctly handle MAKEENTRY.

Suggested by:	Chris Torek <torek@pi-coral.com>
Reviewed by:	mckusick
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks