x86/hyper-v: Fix definition of HV_MAX_FLUSH_REP_COUNT

The max flush rep count of HvFlushGuestPhysicalAddressList hypercall is
equal with how many entries of union hv_gpa_page_range can be populated
into the input parameter page.

The code lacks parenthesis around PAGE_SIZE - 2 * sizeof(u64) which results
in bogus computations. Add them.

Fixes: cc4edae4 ("x86/hyper-v: Add HvFlushGuestAddressList hypercall support")
Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: kys@microsoft.com
Cc: haiyangz@microsoft.com
Cc: sthemmin@microsoft.com
Cc: sashal@kernel.org
Cc: bp@alien8.de
Cc: hpa@zytor.com
Cc: gregkh@linuxfoundation.org
Cc: devel@linuxdriverproject.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190225143114.5149-1-Tianyu.Lan@microsoft.com