lib/percpu_counter: tame kernel-doc compile warning

lib/percpu_counter: tame kernel-doc compile warning

commit a4b35f28c417 ("writeback: rework wb_[dec|inc]_stat family of
functions") add some function description of percpu_counter_add_batch.
but the double '*' in comments means a kernel-doc format comment which
isn't right.

Since the whole file of lib/percpu_counter.c has no any other kernel-doc
format comments, we'd better to remove this incomplete one to tame the
kernel-doc warning:

lib/percpu_counter.c:83: warning: Function parameter or member 'fbc' not described in 'percpu_counter_add_batch'
lib/percpu_counter.c:83: warning: Function parameter or member 'amount' not described in 'percpu_counter_add_batch'
lib/percpu_counter.c:83: warning: Function parameter or member 'batch' not described in 'percpu_counter_add_batch'

Link: https://lkml.kernel.org/r/20210405135505.132446-1-alexs@kernel.org
Signed-off-by: Alex Shi <alexs@kernel.org>
Cc: Nikolay Borisov <nborisov@suse.com>
Cc: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>