CIFS: Return error code when getting file handle for writeback

Now we just return NULL cifsFileInfo pointer in cases we didn't find
or couldn't reopen a file. This hides errors from cifs_reopen_file()
especially retryable errors which should be handled appropriately.
Create new cifs_get_writable_file() routine that returns error codes
from cifs_reopen_file() and use it in the writeback codepath.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsproto.h

@@ -134,6 +134,9 @@ extern bool is_size_safe_to_change(struct cifsInodeInfo *, __u64 eof);
 extern void cifs_update_eof(struct cifsInodeInfo *cifsi, loff_t offset,
 			    unsigned int bytes_written);
 extern struct cifsFileInfo *find_writable_file(struct cifsInodeInfo *, bool);
+extern int cifs_get_writable_file(struct cifsInodeInfo *cifs_inode,
+				  bool fsuid_only,
+				  struct cifsFileInfo **ret_file);
 extern struct cifsFileInfo *find_readable_file(struct cifsInodeInfo *, bool);
 extern unsigned int smbCalcSize(void *buf, struct TCP_Server_Info *server);
 extern int decode_negTokenInit(unsigned char *security_blob, int length,