sctp: fix Kconfig bug in default cookie hmac selection

Commit 0d0863b0 ("sctp: Change defaults on cookie hmac selection")
added a "choice" to the sctp Kconfig file.  It introduced a bug which
led to an infinite loop when while running "make oldconfig".

The problem is that the wrong symbol was defined as the default value
for the choice.  Using the correct value gets rid of the infinite loop.

Note:  if CONFIG_SCTP_COOKIE_HMAC_SHA1=y was present in the input
config file, both that and CONFIG_SCTP_COOKIE_HMAC_MD5=y be present
in the generated config file.
Signed-off-by: Alex Elder <elder@inktank.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/sctp/Kconfig

@@ -68,7 +68,7 @@ config SCTP_DBG_OBJCNT
 	  If unsure, say N
 choice
 	prompt "Default SCTP cookie HMAC encoding"
-	default SCTP_COOKIE_HMAC_MD5
+	default SCTP_DEFAULT_COOKIE_HMAC_MD5
 	help
 	  This option sets the default sctp cookie hmac algorithm
 	  when in doubt select 'md5'