btrfs: move inline function code to header file

When building btrfs from kernel code, it will report:

	fs/btrfs/extent_io.h:281: warning: 'extent_buffer_page' declared inline after being called
	fs/btrfs/extent_io.h:281: warning: previous declaration of 'extent_buffer_page' was here
	fs/btrfs/extent_io.h:280: warning: 'num_extent_pages' declared inline after being called
	fs/btrfs/extent_io.h:280: warning: previous declaration of 'num_extent_pages' was here

because of the wrong declaration of inline functions.
Signed-off-by: Robin Dong <sanbai@taobao.com>

fs/btrfs/extent_io.c

@@ -3986,18 +3986,6 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	return ret;
 }
 
-inline struct page *extent_buffer_page(struct extent_buffer *eb,
-					      unsigned long i)
-{
-	return eb->pages[i];
-}
-
-inline unsigned long num_extent_pages(u64 start, u64 len)
-{
-	return ((start + len + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT) -
-		(start >> PAGE_CACHE_SHIFT);
-}
-
 static void __free_extent_buffer(struct extent_buffer *eb)
 {
 #if LEAK_DEBUG

fs/btrfs/extent_io.h

@@ -282,8 +282,18 @@ void free_extent_buffer_stale(struct extent_buffer *eb);
 int read_extent_buffer_pages(struct extent_io_tree *tree,
 			     struct extent_buffer *eb, u64 start, int wait,
 			     get_extent_t *get_extent, int mirror_num);
-unsigned long num_extent_pages(u64 start, u64 len);
-struct page *extent_buffer_page(struct extent_buffer *eb, unsigned long i);
+
+static inline unsigned long num_extent_pages(u64 start, u64 len)
+{
+	return ((start + len + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT) -
+		(start >> PAGE_CACHE_SHIFT);
+}
+
+static inline struct page *extent_buffer_page(struct extent_buffer *eb,
+					      unsigned long i)
+{
+	return eb->pages[i];
+}
 
 static inline void extent_buffer_get(struct extent_buffer *eb)
 {