xfs: change xfs_xattr_class from a TRACE_EVENT() to DECLARE_EVENT_CLASS()

xfs_xattr_class was accidentally created as a TRACE_EVENT() instead of a
class with DECLARE_EVENT_CLASS().

Note, TRACE_EVENT() is just defined as:

 #define TRACE_EVENT(name, proto, args, tstruct, assign, print) \
	DECLARE_EVENT_CLASS(name,			       \
			     PARAMS(proto),		       \
			     PARAMS(args),		       \
			     PARAMS(tstruct),		       \
			     PARAMS(assign),		       \
			     PARAMS(print));		       \
	DEFINE_EVENT(name, name, PARAMS(proto), PARAMS(args));

The difference between TRACE_EVENT() and DECLARE_EVENT_CLASS() is that
TRACE_EVENT() also creates an event with the class name.

Switch xfs_xattr_class over to being a class and not an event as it is not
called directly, and that event with the class name takes up unnecessary
memory.

Fixes: e47dcf11 ("xfs: repair extended attributes")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>