export: fix string handling of namespace in EXPORT_SYMBOL_NS

export: fix string handling of namespace in EXPORT_SYMBOL_NS

Commit c1391baa006f ("export: avoid code duplication in
include/linux/export.h") broke the ability for a defined string to be
used as a namespace value.  Fix this up by using stringify to properly
encode the namespace name.

Fixes: c1391baa006f ("export: avoid code duplication in include/linux/export.h")
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Quentin Perret <qperret@google.com>
Cc: Matthias Maennich <maennich@google.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220427090442.2105905-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>