net/sock.h: fix sk_peek_off kernel-doc warning

Fix kernel-doc warning in net/sock.h:

Warning(include/net/sock.h:377): No description found for parameter 'sk_peek_off'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>