mac80211: Use GFP_KERNEL instead of GFP_ATOMIC when possible

Previous memory allocations in this function already use GFP_KERNEL, so
use __dev_alloc_skb() and an explicit GFP_KERNEL instead of an implicit
GFP_ATOMIC.

This gives more opportunities of successful allocation.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/194a0e2ff00c3fae88cc9fba47431747360c8242.1645345378.git.christophe.jaillet@wanadoo.frSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh.c

@@ -853,7 +853,7 @@ ieee80211_mesh_build_beacon(struct ieee80211_if_mesh *ifmsh)
 
 	bcn = kzalloc(sizeof(*bcn) + head_len + tail_len, GFP_KERNEL);
 	/* need an skb for IE builders to operate on */
-	skb = dev_alloc_skb(max(head_len, tail_len));
+	skb = __dev_alloc_skb(max(head_len, tail_len), GFP_KERNEL);
 
 	if (!bcn || !skb)
 		goto out_free;