getino: split operation type and namespace type

getino: split operation type and namespace type

Separate the single GETINO_* enum into two levels: a basic operation
type (GETINO_OP_PIDFS vs GETINO_OP_NAMESPACE) and a namespace sub-type
that directly indexes the ns_info[] array starting at 0.

This eliminates the IS_NAMESPACE_OP() macro, the get_pidfd_ns_ioctl()
and pidfd_get_nsfd_or_err() helper functions, and the unused pidfd_ioctl
field from getino_context. The dispatch in print_inode() now uses a
clean switch on ctx->op.

Signed-off-by: Karel Zak <kzak@redhat.com>