bpf: allow ctx writes using BPF_ST_MEM instruction

Lift verifier restriction to use BPF_ST_MEM instructions to write to
context data structures. This requires the following changes:
 - verifier.c:do_check() for BPF_ST updated to:
   - no longer forbid writes to registers of type PTR_TO_CTX;
   - track dst_reg type in the env->insn_aux_data[...].ptr_type field
     (same way it is done for BPF_STX and BPF_LDX instructions).
 - verifier.c:convert_ctx_access() and various callbacks invoked by
   it are updated to handled BPF_ST instruction alongside BPF_STX.

Signed-off-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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