KVM: MMU: make use of ->root_level in reset_rsvds_bits_mask

The reset_rsvds_bits_mask() function can use the guest walker's root level
number instead of using a separate 'level' variable.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Avi Kivity <avi@redhat.com>
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