x86/fpu: Update stale variable name in comment

When the fpu code was reworked pcntxt_mask was renamed to
xfeatures_mask. Reflect it in the comment as well.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191101123850.GE1615@uranus.lan

arch/x86/kernel/fpu/xstate.c

@@ -840,7 +840,7 @@ void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
 
 	/*
 	 * We should not ever be requesting features that we
-	 * have not enabled.  Remember that pcntxt_mask is
+	 * have not enabled.  Remember that xfeatures_mask is
 	 * what we write to the XCR0 register.
 	 */
 	WARN_ONCE(!(xfeatures_mask & BIT_ULL(xfeature_nr)),