Fix three golint warnings

We are clean again.

Warnings were:

internal/fusefrontend/fs.go:443:14: should omit type string from declaration
of var cTarget; it will be inferred from the right-hand side
internal/fusefrontend/xattr.go:26:1: comment on exported method FS.GetXAttr
should be of the form "GetXAttr ..."
internal/syscallcompat/sys_common.go:9:7: exported const PATH_MAX should have
comment or be unexported

internal/fusefrontend/fs.go

@@ -440,7 +440,7 @@ func (fs *FS) Symlink(target string, linkName string, context *fuse.Context) (co
 		return fuse.ToStatus(err)
 	}
 	defer dirfd.Close()
-	var cTarget string = target
+	cTarget := target
 	if !fs.args.PlaintextNames {
 		// Symlinks are encrypted like file contents (GCM) and base64-encoded
 		cTarget = fs.encryptSymlinkTarget(target)

internal/fusefrontend/xattr.go

@@ -23,7 +23,7 @@ var xattrNameIV = []byte("xattr_name_iv_xx")
 // encrypted original name.
 var xattrStorePrefix = "user.gocryptfs."
 
-// GetXAttr: read the value of extended attribute "attr".
+// GetXAttr reads the value of extended attribute "attr".
 // Implements pathfs.Filesystem.
 func (fs *FS) GetXAttr(path string, attr string, context *fuse.Context) ([]byte, fuse.Status) {
 	if fs.isFiltered(path) {

internal/syscallcompat/sys_common.go

@@ -6,7 +6,9 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-const PATH_MAX = 4096 // not defined on Darwin
+// PATH_MAX is the maximum allowed path length on Linux.
+// It is not defined on Darwin, so we use the Linux value.
+const PATH_MAX = 4096
 
 // Readlinkat exists both in Linux and in MacOS 10.10+. We may add an
 // emulated version for users on older MacOS versions if there is