nilfs2: use __field_struct() for a bitwise field

As one can see in include/trace/stages/stage4_event_fields.h, the
implementation of __field() uses the is_signed_type() macro.  As one can
see in commit dcf8e56 ("tracing: Define the is_signed_type() macro
once"), there has been an attempt to not make is_signed_type() trigger
sparse warnings for bitwise types.

Despite that change, sparse complains when passing a bitwise type to
is_signed_type().  The reason is that in its definition below, an
inequality comparison will be made against bitwise types, which are random
collections of bits (the casts to bitwise types themselves are
semantically valid and not problematic):

 #define is_signed_type(type) (((type)(-1)) < (__force type)1)

So, as a workaround, follow the example of <trace/events/initcall.h> and
suppress the following sparse warnings by changing __field() into
__field_struct() that doesn't use is_signed_type():

 fs/nilfs2/segment.c: note: in included file (through
   include/trace/trace_events.h, include/trace/define_trace.h,
   include/trace/events/nilfs2.h):
 ./include/trace/events/nilfs2.h:191:1: warning: cast to restricted
   blk_opf_t
 ./include/trace/events/nilfs2.h:191:1: warning: restricted blk_opf_t
   degrades to integer
 ./include/trace/events/nilfs2.h:191:1: warning: restricted blk_opf_t
   degrades to integer

[konishi.ryusuke: describe the reason for the warnings per Linus's explanation]
  Link: https://lkml.kernel.org/r/20240507222041.4876-1-konishi.ryusuke@gmail.com
Link: https://lkml.kernel.org/r/20240507142454.3344-1-konishi.ryusuke@gmail.com
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401092241.I4mm9OWl-lkp@intel.com/
Reported-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Closes: https://lore.kernel.org/all/20240430080019.4242-2-konishi.ryusuke@gmail.com/
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/trace/events/nilfs2.h

@@ -200,7 +200,11 @@ TRACE_EVENT(nilfs2_mdt_submit_block,
 		    __field(struct inode *, inode)
 		    __field(unsigned long, ino)
 		    __field(unsigned long, blkoff)
-		    __field(enum req_op, mode)
+		    /*
+		     * Use field_struct() to avoid is_signed_type() on the
+		     * bitwise type enum req_op.
+		     */
+		    __field_struct(enum req_op, mode)
 	    ),
 
 	    TP_fast_assign(