-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

There are currently a couple of objects (`req` and `rsp`), in a couple
of structures, that contain flexible structures (`struct l2cap_ecred_conn_req`
and `struct l2cap_ecred_conn_rsp`), for example:

struct l2cap_ecred_rsp_data {
        struct {
                struct l2cap_ecred_conn_rsp rsp;
                __le16 scid[L2CAP_ECRED_MAX_CID];
        } __packed pdu;
        int count;
};

in the struct above, `struct l2cap_ecred_conn_rsp` is a flexible
structure:

struct l2cap_ecred_conn_rsp {
        __le16 mtu;
        __le16 mps;
        __le16 credits;
        __le16 result;
        __le16 dcid[];
};

So, in order to avoid ending up with a flexible-array member in the
middle of another structure, we use the `struct_group_tagged()` (and
`__struct_group()` when the flexible structure is `__packed`) helper
to separate the flexible array from the rest of the members in the
flexible structure:

struct l2cap_ecred_conn_rsp {
        struct_group_tagged(l2cap_ecred_conn_rsp_hdr, hdr,

	... the rest of members

        );
        __le16 dcid[];
};

With the change described above, we now declare objects of the type of
the tagged struct, in this example `struct l2cap_ecred_conn_rsp_hdr`,
without embedding flexible arrays in the middle of other structures:

struct l2cap_ecred_rsp_data {
        struct {
                struct l2cap_ecred_conn_rsp_hdr rsp;
                __le16 scid[L2CAP_ECRED_MAX_CID];
        } __packed pdu;
        int count;
};

Also, when the flexible-array member needs to be accessed, we use
`container_of()` to retrieve a pointer to the flexible structure.

We also use the `DEFINE_RAW_FLEX()` helper for a couple of on-stack
definitions of a flexible structure where the size of the flexible-array
member is known at compile-time.

So, with these changes, fix the following warnings:
net/bluetooth/l2cap_core.c:1260:45: warning: structure containing a
flexible array member is not at the end of another structure
[-Wflex-array-member-not-at-end]
net/bluetooth/l2cap_core.c:3740:45: warning: structure containing a
flexible array member is not at the end of another structure
[-Wflex-array-member-not-at-end]
net/bluetooth/l2cap_core.c:4999:45: warning: structure containing a
flexible array member is not at the end of another structure
[-Wflex-array-member-not-at-end]
net/bluetooth/l2cap_core.c:7116:47: warning: structure containing a
flexible array member is not at the end of another structure
[-Wflex-array-member-not-at-end]

Link: https://github.com/KSPP/linux/issues/202Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>