Add FEATURE knob for testing for UFS extended attribute kernel support

Support can be verified via `feature_present("ufs_extattr")`, etc.

Differential Revision: https://reviews.freebsd.org/D6053
MFC after: 2 weeks
Relnotes: yes
Reviewed by: asomers, kib
Sponsored by: EMC / Isilon Storage Division

sys/ufs/ufs/ufs_extattr.c

@@ -69,6 +69,8 @@ __FBSDID("$FreeBSD$");
 
 #ifdef UFS_EXTATTR
 
+FEATURE(ufs_extattr, "ufs extended attribute support");
+
 static MALLOC_DEFINE(M_UFS_EXTATTR, "ufs_extattr", "ufs extended attribute");
 
 static int ufs_extattr_sync = 0;