binfmt_elf_fdpic: fix execfd build regression

The change to bprm->have_execfd was incomplete, leading
to a build failure:

fs/binfmt_elf_fdpic.c: In function 'create_elf_fdpic_tables':
fs/binfmt_elf_fdpic.c:591:27: error: 'BINPRM_FLAGS_EXECFD' undeclared

Change the last user of BINPRM_FLAGS_EXECFD in a corresponding
way.

Reported-by: Valdis Klētnieks <valdis.kletnieks@vt.edu>
Fixes: b8a61c9e

 ("exec: Generic execfd support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>