os: make return type of FileInfo.Permission() consistent with related functions

(uint32 rather than int)

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/1958048

src/pkg/os/types.go

@@ -53,4 +53,4 @@ func (f *FileInfo) IsSymlink() bool { return (f.Mode & syscall.S_IFMT) == syscal
 func (f *FileInfo) IsSocket() bool { return (f.Mode & syscall.S_IFMT) == syscall.S_IFSOCK }
 
 // Permission returns the file permission bits.
-func (f *FileInfo) Permission() int { return int(f.Mode & 0777) }
+func (f *FileInfo) Permission() uint32 { return f.Mode & 0777 }