um: Make CONFIG_STATIC_LINK actually static

Currently, CONFIG_STATIC_LINK can be enabled with options which cannot
be statically linked, namely UML_NET_VECTOR, UML_NET_VDE, and
UML_NET_PCAP; this is because glibc tries to load NSS which does not
support being statically linked. So make CONFIG_STATIC_LINK depend on
!UML_NET_VECTOR && !UML_NET_VDE && !UML_NET_PCAP.

Link: https://lore.kernel.org/lkml/f658f317-be54-ed75-8296-c373c2dcc697@cambridgegreys.com/#t
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/Kconfig

@@ -62,9 +62,12 @@ config NR_CPUS
 
 source "arch/$(HEADER_ARCH)/um/Kconfig"
 
+config FORBID_STATIC_LINK
+	bool
+
 config STATIC_LINK
 	bool "Force a static link"
-	default n
+	depends on !FORBID_STATIC_LINK
 	help
 	  This option gives you the ability to force a static link of UML.
 	  Normally, UML is linked as a shared binary.  This is inconvenient for
@@ -73,6 +76,9 @@ config STATIC_LINK
 	  Additionally, this option enables using higher memory spaces (up to
 	  2.75G) for UML.
 
+	  NOTE: This option is incompatible with some networking features which
+	  depend on features that require being dynamically loaded (like NSS).
+
 config LD_SCRIPT_STATIC
 	bool
 	default y

arch/um/drivers/Kconfig

@@ -234,6 +234,7 @@ config UML_NET_DAEMON
 config UML_NET_VECTOR
 	bool "Vector I/O high performance network devices"
 	depends on UML_NET
+	select FORBID_STATIC_LINK
 	help
 	This User-Mode Linux network driver uses multi-message send
 	and receive functions. The host running the UML guest must have
@@ -245,6 +246,7 @@ config UML_NET_VECTOR
 config UML_NET_VDE
 	bool "VDE transport (obsolete)"
 	depends on UML_NET
+	select FORBID_STATIC_LINK
 	help
 	This User-Mode Linux network transport allows one or more running
 	UMLs on a single host to communicate with each other and also
@@ -292,6 +294,7 @@ config UML_NET_MCAST
 config UML_NET_PCAP
 	bool "pcap transport (obsolete)"
 	depends on UML_NET
+	select FORBID_STATIC_LINK
 	help
 	The pcap transport makes a pcap packet stream on the host look
 	like an ethernet device inside UML.  This is useful for making