Add a tunable to disable BRT support.

Copy the disable parameter that FreeBSD implemented, and extend it to
work on Linux as well, until we're sure this is stable.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #15529

include/os/linux/zfs/sys/zfs_vfsops_os.h

@@ -45,6 +45,8 @@ extern "C" {
 typedef struct zfsvfs zfsvfs_t;
 struct znode;
 
+extern int zfs_bclone_enabled;
+
 /*
  * This structure emulates the vfs_t from other platforms.  It's purpose
  * is to facilitate the handling of mount options and minimize structural