kbuild: fix installing external modules

Eric Sandeen <sandeen@redhat.com> reported:
  Installing external modules is supposed to put them in some path
  under /lib/modules/<version>/extra/subdir/, but this change:
  http://linux.bkbits.net:8080/linux-2.6/?PAGE=cset&REV=1.1982.9.23
  makes them go under /lib/modules/<version>/extrasubdir

 (for example, make M=fs/ext3 modules_install puts ext3.ko in
 /lib/modules/<version>/extrafs/ext3.ko)

This was the case only when specifying a trailing slash to M=..

Fixed by removing trailing slash if present so
we correctly match dir part of target.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Eric Sandeen <sandeen@redhat.com>

scripts/Makefile.modinst

@@ -21,7 +21,7 @@ quiet_cmd_modules_install = INSTALL $@
 
 # Modules built outside the kernel source tree go into extra by default
 INSTALL_MOD_DIR ?= extra
-ext-mod-dir = $(INSTALL_MOD_DIR)$(subst $(KBUILD_EXTMOD),,$(@D))
+ext-mod-dir = $(INSTALL_MOD_DIR)$(subst $(patsubst %/,%,$(KBUILD_EXTMOD)),,$(@D))
 
 modinst_dir = $(if $(KBUILD_EXTMOD),$(ext-mod-dir),kernel/$(@D))