Revert "Remove TSD zfs_fsyncer_key"

This reverts commit 31f2b5abdf back
to the original code until the fsync(2) performance regression
can be addressed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/zfs_vfsops.h

@@ -150,6 +150,8 @@ typedef struct zfid_long {
 #define	SHORT_FID_LEN	(sizeof (zfid_short_t) - sizeof (uint16_t))
 #define	LONG_FID_LEN	(sizeof (zfid_long_t) - sizeof (uint16_t))
 
+extern uint_t zfs_fsyncer_key;
+
 extern int zfs_suspend_fs(zfs_sb_t *zsb);
 extern int zfs_resume_fs(zfs_sb_t *zsb, const char *osname);
 extern int zfs_userspace_one(zfs_sb_t *zsb, zfs_userquota_prop_t type,