Btrfs: The file argument for fsync() is never null

The "file" argument for fsync is never null so we can remove this check.

What drew my attention here is that 7ea80859: "drop unused dentry
argument to ->fsync" introduced an unconditional dereference at the
start of the function and that generated a smatch warning.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>