debugfs: Make debugfs_cancellation a context lock struct

debugfs: Make debugfs_cancellation a context lock struct

When compiling include/linux/debugfs.h with CONTEXT_ANALYSIS enabled, we
can see this error:

./include/linux/debugfs.h:239:17: error: use of undeclared identifier 'cancellation'
  239 | void __acquires(cancellation)

Move the __acquires(..) attribute after the declaration, so that the
compiler can see the cancellation function argument, as well as making
struct debugfs_cancellation a real context lock to benefit from Clang's
context analysis.

This change is a preparatory change to allow enabling context analysis
in subsystems that include the above header.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20251219154418.3592607-22-elver@google.com