scoutfs: add percpu coutners with sysfs files

Add percpu counters that will let us track all manner of things.

To report them we add a sysfs directory full of attribute files in a
sysfs dir for each mount:

    # (cd /sys/fs/scoutfs/loop0/counters && grep . *)
    skip_delete:0
    skip_insert:3218
    skip_lookup:8439
    skip_next:1190
    skip_search:156

The implementation is careful to define each counter in only one place.
We don't have to make sure that a bunch of defintions and arrays are in
sync.

This builds off of Ben's initial patches that added sysfs dirs.

Signed-off-by: Zach Brown <zab@versity.com>
Signed-off-by: Ben McClelland <ben.mcclelland@versity.com>

kmod/src/Makefile

@@ -2,5 +2,5 @@ obj-$(CONFIG_SCOUTFS_FS) := scoutfs.o
 
 CFLAGS_scoutfs_trace.o = -I$(src) # define_trace.h double include
  
-scoutfs-y += block.o bloom.o chunk.o crc.o dir.o filerw.o inode.o manifest.o \
-	     msg.o ring.o scoutfs_trace.o segment.o skip.o super.o
+scoutfs-y += block.o bloom.o counters.o chunk.o crc.o dir.o filerw.o inode.o \
+	     manifest.o msg.o ring.o scoutfs_trace.o segment.o skip.o super.o