btrfs: speed up btrfs_get_##bits helpers

The helpers unconditionally call map_private_extent_buffer to get the
address of page containing the requested offset plus the mapping start
and length. Depending on the return value, the fast path uses unaligned
read to get data within a page, or fall back to read_extent_buffer that
can handle reads spanning more pages.

This is all wasteful. We know the number of bytes to read, 1/2/4/8 and
can find out the page. Then simply check if it's contained or the
fallback is needed.

This saves one function call to map_private_extent_buffer and several
unnecessary temporary variables.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/struct-funcs.c

@@ -99,28 +99,19 @@ u##bits btrfs_get_token_##bits(struct btrfs_map_token *token,		\
 u##bits btrfs_get_##bits(const struct extent_buffer *eb,		\
 			 const void *ptr, unsigned long off)		\
 {									\
-	unsigned long part_offset = (unsigned long)ptr;			\
-	unsigned long offset = part_offset + off;			\
-	void *p;							\
-	int err;							\
-	char *kaddr;							\
-	unsigned long map_start;					\
-	unsigned long map_len;						\
-	int size = sizeof(u##bits);					\
-	u##bits res;							\
-									\
-	ASSERT(check_setget_bounds(eb, ptr, off, size));		\
-	err = map_private_extent_buffer(eb, offset, size,		\
-					&kaddr, &map_start, &map_len);	\
-	if (err) {							\
+	const unsigned long member_offset = (unsigned long)ptr + off;	\
+	const unsigned long oip = offset_in_page(member_offset);	\
+	const int size = sizeof(u##bits);				\
 	__le##bits leres;						\
 									\
-		read_extent_buffer(eb, &leres, offset, size);		\
-		return le##bits##_to_cpu(leres);			\
+	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]);	\
+		return get_unaligned_le##bits(kaddr + oip);		\
 	}								\
-	p = kaddr + part_offset - map_start;				\
-	res = get_unaligned_le##bits(p + off);				\
-	return res;							\
+	read_extent_buffer(eb, &leres, member_offset, size);		\
+	return le##bits##_to_cpu(leres);				\
 }									\
 void btrfs_set_token_##bits(struct btrfs_map_token *token,		\
 			    const void *ptr, unsigned long off,		\