net: pkt: Introduce minimum length requirement to net_pkt_get_frag()

net_pkt_get_frag() and a few other functions did not specify the
allocated fragment length, incorrectly assuming that fixed-sized
buffers are always used.

In order to make the function work properly also with variable-sized
buffers, extend the function argument list with minimum expected
fragment length parameter. This allows to use net_buf_alloc_len()
allocator in variable buffer length configuration, as well as verify if
the fixed-sized buffer is large enough to satisfy the requirements
otherwise.

Update the existing codebase to provide the expected fragment length,
based on the context.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>