arm64: mm: remove broken &= operator from pmd_mknotpresent

This should be a plain old '&' and could easily lead to undefined
behaviour if the target of a pmd_mknotpresent invocation was the same
as the parameter.

Fixes: 9c7e535fcc17 (arm64: mm: Route pmd thp functions through pte equivalents)
Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: <stable@vger.kernel.org> # v3.15
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/pgtable.h

@@ -246,7 +246,7 @@ static inline pmd_t pte_pmd(pte_t pte)
 #define pmd_mkwrite(pmd)	pte_pmd(pte_mkwrite(pmd_pte(pmd)))
 #define pmd_mkdirty(pmd)	pte_pmd(pte_mkdirty(pmd_pte(pmd)))
 #define pmd_mkyoung(pmd)	pte_pmd(pte_mkyoung(pmd_pte(pmd)))
-#define pmd_mknotpresent(pmd)	(__pmd(pmd_val(pmd) &= ~PMD_TYPE_MASK))
+#define pmd_mknotpresent(pmd)	(__pmd(pmd_val(pmd) & ~PMD_TYPE_MASK))
 
 #define __HAVE_ARCH_PMD_WRITE
 #define pmd_write(pmd)		pte_write(pmd_pte(pmd))