This callback allows use-cases where the SDU is much larger than the l2cap
MPS. The stack will then try to allocate using this callback if specified,
and fall-back on using the buffer's pool (previous behavior).
This way one can define two buffer pools, one with a very large buffer
size, and one with a buffer size >= MPS, and the stack will allocate from
that instead.
Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
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