Revert __LINUX_SECURITY_H: breaks compilation on kernel 6.17

linux/perf_event.h calls security_perf_event_open() which is declared in
linux/security.h. Blocking security.h via __LINUX_SECURITY_H therefore
breaks BPF compilation on kernel 6.17 — confirmed on de-dev1.dlpxdc.co
(2.4.99-1delphix.2026.03.15, kernel 6.17.0-1007).

The kernel 6.14 bpf_wq compile error (DLPX-96701 class) needs a
kernel-version-conditional fix in estat.py rather than a blanket
header guard in the BPF source.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: dbshah12

bpf/estat/metaslab-alloc.c

@@ -4,15 +4,6 @@
  * SPDX-License-Identifier: GPL-2.0-or-later
  */
 
-/*
- * On some kernel versions (e.g. 6.14) linux/bpf.h is pulled in transitively
- * via metaslab.h -> spa.h -> zfs_context.h -> vfs.h -> security.h and
- * contains sizeof(struct bpf_wq) where bpf_wq is only forward-declared,
- * causing a compile error (DLPX-96701).  Blocking security.h (which this BPF
- * program does not need) stops the chain without touching linux/bpf.h itself.
- */
-#define __LINUX_SECURITY_H
-
 #include <sys/metaslab.h>
 #include <sys/metaslab_impl.h>
 #include <sys/vdev_impl.h>