UPSTREAM: seccomp: document the "filter_count" field

Add missing kernel-doc for the 'filter_count' field in struct seccomp.

seccomp.h:40: warning: Function parameter or member 'filter_count' not described in 'seccomp'

Bug: 254441685
Fixes: c818c03b ("seccomp: Report number of loaded filters in /proc/$pid/status")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221202073953.14677-1-rdunlap@infradead.org


(cherry picked from commit b9069728)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: Iea933bccd1f2bd4365ccc7a7c22c5b1c093c6a70