mm: fs: initialize fsdata passed to write_begin/write_end interface

Functions implementing the a_ops->write_end() interface accept the
`void *fsdata` parameter that is supposed to be initialized by the
corresponding a_ops->write_begin() (which accepts `void **fsdata`).

However not all a_ops->write_begin() implementations initialize `fsdata`
unconditionally, so it may get passed uninitialized to a_ops->write_end(),
resulting in undefined behavior.

Fix this by initializing fsdata with NULL before the call to
write_begin(), rather than doing so in all possible a_ops
implementations.

This patch covers only the following cases found by running x86 KMSAN
under syzkaller:

 - generic_perform_write()
 - cont_expand_zero() and generic_cont_expand_simple()
 - page_symlink()

Other cases of passing uninitialized fsdata may persist in the codebase.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
Link: https://linux-review.googlesource.com/id/Ie300c21bbe9dea69a730745bd3c6d2720953bf41

fs/buffer.c

@@ -2352,7 +2352,7 @@ int generic_cont_expand_simple(struct inode *inode, loff_t size)
 	struct address_space *mapping = inode->i_mapping;
 	const struct address_space_operations *aops = mapping->a_ops;
 	struct page *page;
-	void *fsdata;
+	void *fsdata = NULL;
 	int err;
 
 	err = inode_newsize_ok(inode, size);
@@ -2378,7 +2378,7 @@ static int cont_expand_zero(struct file *file, struct address_space *mapping,
 	const struct address_space_operations *aops = mapping->a_ops;
 	unsigned int blocksize = i_blocksize(inode);
 	struct page *page;
-	void *fsdata;
+	void *fsdata = NULL;
 	pgoff_t index, curidx;
 	loff_t curpos;
 	unsigned zerofrom, offset, len;

fs/namei.c

@@ -5088,7 +5088,7 @@ int page_symlink(struct inode *inode, const char *symname, int len)
 	const struct address_space_operations *aops = mapping->a_ops;
 	bool nofs = !mapping_gfp_constraint(mapping, __GFP_FS);
 	struct page *page;
-	void *fsdata;
+	void *fsdata = NULL;
 	int err;
 	unsigned int flags;
 

mm/filemap.c

@@ -3712,7 +3712,7 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i)
 		unsigned long offset;	/* Offset into pagecache page */
 		unsigned long bytes;	/* Bytes to write to page */
 		size_t copied;		/* Bytes copied from user */
-		void *fsdata;
+		void *fsdata = NULL;
 
 		offset = (pos & (PAGE_SIZE - 1));
 		bytes = min_t(unsigned long, PAGE_SIZE - offset,