fix allmodconfig breakage

If you use KCONFIG_ALLCONFIG (even with empty file) you get broken
allmodconfig/allyesconfig; CONFIG_MODULES gets turned off, with obvious
massive fallout.

Breakage had been introduced when conf_set_all_new_symbols() got used
for allmodconfig et.al.

What happens is that sym_calc_value(modules_sym) done in
conf_read_simple() sets SYMBOL_VALID on both modules_sym and MODULES.
When we get to conf_set_all_new_symbols(), we set sym->def[S_DEF_USER]
on everything, but it has no effect on sym->curr for the symbols that
already have SYMBOL_VALID - these are stuck.

Solution: use sym_clear_all_valid() in there.  Note that it makes
reevaluation of modules_sym redundant - sym_clear_all_valid() will do
that itself.

[ Fixes http://bugzilla.kernel.org/show_bug.cgi?id=11512

, says Alexey ]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>