btrfs: optimize split page read in btrfs_get_##bits

The helper read_extent_buffer is called to do read of the data spanning
two extent buffer pages. As the size is known, we can do the read
directly in two steps.  This removes one function call and compiler can
optimize memcpy as the sizes are known at compile time.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/struct-funcs.c

@@ -90,17 +90,20 @@ u##bits btrfs_get_##bits(const struct extent_buffer *eb,		\
 {									\
 	const unsigned long member_offset = (unsigned long)ptr + off;	\
 	const unsigned long oip = offset_in_page(member_offset);	\
+	const unsigned long idx = member_offset >> PAGE_SHIFT;		\
+	char *kaddr = page_address(eb->pages[idx]);			\
 	const int size = sizeof(u##bits);				\
-	__le##bits leres;						\
+	const int part = PAGE_SIZE - oip;				\
+	u8 lebytes[sizeof(u##bits)];					\
 									\
 	ASSERT(check_setget_bounds(eb, ptr, off, size));		\
-	if (oip + size <= PAGE_SIZE) {					\
-		const unsigned long idx = member_offset >> PAGE_SHIFT;	\
-		const char *kaddr = page_address(eb->pages[idx]);	\
+	if (oip + size <= PAGE_SIZE)					\
 		return get_unaligned_le##bits(kaddr + oip);		\
-	}								\
-	read_extent_buffer(eb, &leres, member_offset, size);		\
-	return le##bits##_to_cpu(leres);				\
+									\
+	memcpy(lebytes, kaddr + oip, part);				\
+	kaddr = page_address(eb->pages[idx + 1]);			\
+	memcpy(lebytes + part, kaddr, size - part);			\
+	return get_unaligned_le##bits(lebytes);				\
 }									\
 void btrfs_set_token_##bits(struct btrfs_map_token *token,		\
 			    const void *ptr, unsigned long off,		\