libio: Fix oversized __io_vtables

IO_VTABLES_LEN is the size of the struct array in bytes, not the number
of __IO_jump_t's in the array. Drops just under 384kb from .rodata on
LP64 machines.

Fixes: 3020f726 ("libio: Remove the usage of __libc_IO_vtables")
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
Tested-by: Florian Weimer <fweimer@redhat.com>