bpf: BPF token support for BPF_BTF_GET_FD_BY_ID

Currently BPF_BTF_GET_FD_BY_ID requires CAP_SYS_ADMIN, which does not
allow running it from user namespace. This creates a problem when
freplace program running from user namespace needs to query target
program BTF.
This patch relaxes capable check from CAP_SYS_ADMIN to CAP_BPF and adds
support for BPF token that can be passed in attributes to syscall.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250317174039.161275-2-mykyta.yatsenko5@gmail.com

include/uapi/linux/bpf.h

@@ -1652,6 +1652,7 @@ union bpf_attr {
 		};
 		__u32		next_id;
 		__u32		open_flags;
+		__s32		fd_by_id_token_fd;
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_GET_INFO_BY_FD */