bpf: Factor out check_atomic_rmw()

bpf: Factor out check_atomic_rmw()

Currently, check_atomic() only handles atomic read-modify-write (RMW)
instructions.  Since we are planning to introduce other types of atomic
instructions (i.e., atomic load/store), extract the existing RMW
handling logic into its own function named check_atomic_rmw().

Remove the @insn_idx parameter as it is not really necessary.  Use
'env->insn_idx' instead, as in other places in verifier.c.

Signed-off-by: Peilin Ye <yepeilin@google.com>
Link: https://lore.kernel.org/r/6323ac8e73a10a1c8ee547c77ed68cf8eb6b90e1.1740978603.git.yepeilin@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>