mac80211: move WEP tailroom size check

mac80211: move WEP tailroom size check

commit 47b4e1fc4972cc43a19121bc2608a60aef3bf216 upstream.

Remove checking tailroom when adding IV as it uses only
headroom, and move the check to the ICV generation that
actually needs the tailroom.

In other case I hit such warning and datapath don't work,
when testing:
- IBSS + WEP
- ath9k with hw crypt enabled
- IPv6 data (ping6)

WARNING: CPU: 3 PID: 13301 at net/mac80211/wep.c:102 ieee80211_wep_add_iv+0x129/0x190 [mac80211]()
[...]
Call Trace:
[<ffffffff817bf491>] dump_stack+0x45/0x57
[<ffffffff8107746a>] warn_slowpath_common+0x8a/0xc0
[<ffffffff8107755a>] warn_slowpath_null+0x1a/0x20
[<ffffffffc09ae109>] ieee80211_wep_add_iv+0x129/0x190 [mac80211]
[<ffffffffc09ae7ab>] ieee80211_crypto_wep_encrypt+0x6b/0xd0 [mac80211]
[<ffffffffc09d3fb1>] invoke_tx_handlers+0xc51/0xf30 [mac80211]
[...]

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.2: s/IEEE80211_WEP/WEP/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>