net: only use a single page of slop in MAX_SKB_FRAGS

In order to accommodate a 64K buffer we need 64K/PAGE_SIZE plus one more page
in order to allow for a buffer which does not start on a page boundary.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>