Use percpu_counter for obj_alloc counter of Linux-backed caches

A previous commit enabled the tracking of object allocations
in Linux-backed caches from the SPL layer for debuggability.
The commit is: 9a170fc6fe54f1e852b6c39630fe5ef2bbd97c16

Unfortunately, it also introduced minor performance regressions
that were highlighted by the ZFS perf test-suite. Within Delphix
we found that the regression would be from -1%, all the way up
to -8% for some workloads.

This commit brings performance back up to par by creating a
separate counter for those caches and making it a percpu in
order to avoid lock-contention.

The initial performance testing was done by myself, and the
final round was conducted by @tonynguien who was also the one
that discovered the regression and highlighted the culprit.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #10397

include/os/linux/spl/sys/kmem_cache.h

@@ -202,6 +202,7 @@ typedef struct spl_kmem_cache {
 	uint64_t		skc_slab_max;	/* Slab max historic  */
 	uint64_t		skc_obj_total;	/* Obj total current */
 	uint64_t		skc_obj_alloc;	/* Obj alloc current */
+	struct percpu_counter	skc_linux_alloc;   /* Linux-backed Obj alloc  */
 	uint64_t		skc_obj_max;	/* Obj max historic */
 	uint64_t		skc_obj_deadlock;  /* Obj emergency deadlocks */
 	uint64_t		skc_obj_emergency; /* Obj emergency current */