modpost: handle huge numbers of modules.

strace shows:
72102 execve("/bin/sh", ["/bin/sh", "-c", "echo '  scripts/mod/modpost -m -a
-o /cc/wfg/sound-compiletest/Module.symvers      -s'; scripts/
mod/modpost -m -a -o /cc/wfg/sound-compiletest/Module.symvers      -s vmlinux
arch/x86/crypto/ablk_helper.o arch/x86/crypto/aes-i586.o arch
/x86/crypto/aesni-intel.o arch/x86/crypto/crc32-pclmul.o
...
drivers/ata/sata_promise.o "...], [/* 119 vars */] <unfinished ...>
71827 wait4(-1,  <unfinished ...>
72102 <... execve resumed> )            = -1 E2BIG (Argument list too long)

So we re-run the shell command which produces the list and feed it into modpost -T -.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

scripts/Makefile.modpost

@@ -60,7 +60,8 @@ kernelsymfile := $(objtree)/Module.symvers
 modulesymfile := $(firstword $(KBUILD_EXTMOD))/Module.symvers
 
 # Step 1), find all modules listed in $(MODVERDIR)/
-__modules := $(sort $(shell grep -h '\.ko$$' /dev/null $(wildcard $(MODVERDIR)/*.mod)))
+MODLISTCMD := find $(MODVERDIR) -name '*.mod' | xargs -r grep -h '\.ko$$' | sort -u
+__modules := $(shell $(MODLISTCMD))
 modules   := $(patsubst %.o,%.ko, $(wildcard $(__modules:.ko=.o)))
 
 # Stop after building .o files if NOFINAL is set. Makes compile tests quicker
@@ -78,12 +79,13 @@ modpost = scripts/mod/modpost                    \
  $(if $(CONFIG_DEBUG_SECTION_MISMATCH),,-S)      \
  $(if $(KBUILD_EXTMOD)$(KBUILD_MODPOST_WARN),-w)
 
+# We can go over command line length here, so be careful.
 quiet_cmd_modpost = MODPOST $(words $(filter-out vmlinux FORCE, $^)) modules
-      cmd_modpost = $(modpost) -s
+      cmd_modpost = $(MODLISTCMD) | sed 's/\.ko$$/.o/' | $(modpost) -s -T -
 
 PHONY += __modpost
 __modpost: $(modules:.ko=.o) FORCE
-	$(call cmd,modpost) $(wildcard vmlinux) $(filter-out FORCE,$^)
+	$(call cmd,modpost) $(wildcard vmlinux)
 
 quiet_cmd_kernel-mod = MODPOST $@
       cmd_kernel-mod = $(modpost) $@