mm: add pmd_t initializer __pmd() to work around a GCC bug.

THP migration is added but only supports x86_64 at the moment. For all
other architectures, swp_entry_to_pmd() only returns a zero pmd_t.

Due to a GCC zero initializer bug #53119, the standard (pmd_t){0}
initializer is not accepted by all GCC versions. __pmd() is a feasible
workaround. In addition, sparc32's pmd_t is an array instead of a single
value, so we need (pmd_t){ {0}, } instead of (pmd_t){0}. Thus,
a different __pmd() definition is needed in sparc32.
Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/include/asm/page_32.h

@@ -68,6 +68,7 @@ typedef struct { unsigned long iopgprot; } iopgprot_t;
 #define iopgprot_val(x)	((x).iopgprot)
 
 #define __pte(x)	((pte_t) { (x) } )
+#define __pmd(x)	((pmd_t) { { (x) }, })
 #define __iopte(x)	((iopte_t) { (x) } )
 #define __pgd(x)	((pgd_t) { (x) } )
 #define __ctxd(x)	((ctxd_t) { (x) } )
@@ -95,6 +96,7 @@ typedef unsigned long iopgprot_t;
 #define iopgprot_val(x)	(x)
 
 #define __pte(x)	(x)
+#define __pmd(x)	((pmd_t) { { (x) }, })
 #define __iopte(x)	(x)
 #define __pgd(x)	(x)
 #define __ctxd(x)	(x)