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