added sftp client implementation

vendor/github.com/pkg/sftp/server_statvfs_linux.go

@@ -0,0 +1,22 @@
+// +build linux
+
+package sftp
+
+import (
+	"syscall"
+)
+
+func statvfsFromStatfst(stat *syscall.Statfs_t) (*StatVFS, error) {
+	return &StatVFS{
+		Bsize:   uint64(stat.Bsize),
+		Frsize:  uint64(stat.Frsize),
+		Blocks:  stat.Blocks,
+		Bfree:   stat.Bfree,
+		Bavail:  stat.Bavail,
+		Files:   stat.Files,
+		Ffree:   stat.Ffree,
+		Favail:  stat.Ffree,         // not sure how to calculate Favail
+		Flag:    uint64(stat.Flags), // assuming POSIX?
+		Namemax: uint64(stat.Namelen),
+	}, nil
+}