fix alloc_skb comment typo

Index: linux-2.6/include/linux/skbuff.h
===================================================================
@@ -1081,7 +1081,7 @@
  *	the headroom they think they need without accounting for the
  *	built in space. The built in space is used for optimisations.
  *
- *	%NULL is returned in there is no free memory.
+ *	%NULL is returned if there is no free memory.
  */
 static inline struct sk_buff *__dev_alloc_skb(unsigned int length,
 					      gfp_t gfp_mask)

@@ -1101,7 +1101,7 @@
  *	the headroom they think they need without accounting for the
  *	built in space. The built in space is used for optimisations.
  *
- *	%NULL is returned in there is no free memory. Although this function
+ *	%NULL is returned if there is no free memory. Although this function
  *	allocates memory it can be called from an interrupt.
  */
 static inline struct sk_buff *dev_alloc_skb(unsigned int length)