Fix: prevent panic when swap file creation fails (#7957)

* Fix: prevent panic when swap file creation fails

* weed mount: fix race condition in swap file initialization

Ensure thread-safe access to sf.file and other state in NewSwapFileChunk
and FreeResource by using sf.chunkTrackingLock consistently. Also
set sf.file to nil after closing to prevent reuse.

* weed mount: improve swap directory creation logic

- Check error for os.MkdirAll and log it if it fails.
- Use 0700 permissions for the swap directory for better security.
- Improve error logging context.

* weed mount: add unit tests for swap file creation

Add tests to verify:
- Concurrent initialization of the swap file.
- Correct directory permissions (0700).
- Automatic directory recreation if deleted.

* weed mount: fix thread-safety in swap file unit tests

Use atomic.Uint32 to track failures within goroutines in
TestSwapFile_NewSwapFileChunk_Concurrent to avoid unsafe calls to
t.Errorf from multiple goroutines.

* weed mount: simplify swap file creation logic

Refactor the directory check and retry logic for better readability and
to avoid re-using the main error variable for directory creation errors.
Remove redundant error logging.

* weed mount: improve error checking in swap file tests

Explicitly check if NewSwapFileChunk returns nil to provide more
informative failures.

* weed mount: update DirtyPages interface to return error

Propagate errors from SaveDataAt when swap file creation fails. This
prevents potential panics in the write path.

* weed mount: handle AddPage errors in write paths

Update ChunkedDirtyPages and PageWriter to propagate errors and update
WFS.Write and WFS.CopyFileRange to return fuse.EIO on failure.

* weed mount: update swap directory creation error message

Change "recreate" to "create/recreate" to better reflect that this path
is also taken during the initial creation of the swap directory.

---------

Co-authored-by: lixiang58 <lixiang58@lenovo.com>
Co-authored-by: Chris Lu <chris.lu@gmail.com>

weed/mount/page_writer/dirty_pages.go

@@ -1,7 +1,7 @@
 package page_writer
 
 type DirtyPages interface {
-	AddPage(offset int64, data []byte, isSequential bool, tsNs int64)
+	AddPage(offset int64, data []byte, isSequential bool, tsNs int64) error
 	FlushData() error
 	ReadDirtyDataAt(data []byte, startOffset int64, tsNs int64) (maxStop int64)
 	Destroy()