The hcall tracing code has a recursion check built in, which skips
tracing if we are already tracing an hcall.

However if the tracing code has problems with recursion, this check
may not catch all cases because the tracing code could be invoked from
a different tracepoint first, then make an hcall that gets traced,
then recurse.

Add an explicit warning if recursion is detected here, which might help
to notice tracing code making hcalls. Really the core trace code should
have its own recursion checking and warnings though.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210508101455.1578318-5-npiggin@gmail.com