exec.c: add a check between constants to see whether we could skip

The maximum level is defined as P_L2_LEVELS and skip is defined with 6
bits, which means if P_L2_LEVELS < (1 << 6), skip never exceeds the
boundary.

Since this check is between two constants, which leverages compiler
to optimize the code based on different configuration.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190321082555.21118-7-richardw.yang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>