kbuild: fix make mrproper for Documentation/DocBook/man

"make mandocs" generate > 2000 files in Documentation/DocBook/man
and this caused kbuild to barf out during make mrproper like this:

make -f scripts/Makefile.clean obj=Documentation/DocBook
make -f scripts/Makefile.clean obj=Documentation/DocBook/man/
make[2]: execvp: /bin/sh: Argument list too long
make[2]: *** [__clean] Error 127
make[1]: *** [Documentation/DocBook/man/] Error 2
make: *** [_mrproper_Documentation/DocBook] Error 2

The man directory were solely used for output
so the fix is to remove it entirely during the
make mrproper process.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>

Documentation/DocBook/Makefile

@@ -152,6 +152,7 @@ quiet_cmd_db2man = MAN     $@
 	@(which xmlto > /dev/null 2>&1) || \
 	 (echo "*** You need to install xmlto ***"; \
 	  exit 1)
+	$(Q)mkdir -p $(obj)/man
 	$(call cmd,db2man)
 	@touch $@
 
@@ -212,11 +213,7 @@ clean-files := $(DOCBOOKS) \
 	$(patsubst %.xml, %.9,    $(DOCBOOKS)) \
 	$(C-procfs-example)
 
-clean-dirs := $(patsubst %.xml,%,$(DOCBOOKS))
-
-#man put files in man subdir - traverse down
-subdir- := man/
-
+clean-dirs := $(patsubst %.xml,%,$(DOCBOOKS)) man
 
 # Declare the contents of the .PHONY variable as phony.  We keep that
 # information in a variable se we can use it in if_changed and friends.