libpod: Factor out usage of unix.MNT_DETACH from (*Volume).unmount

There is an existing wrapper for unix.Unmount(..., MNT_DETACH) in
util_linux.go but that filters all errors and for volumes, we only want
to filter EINVAL. The existing libpod.Unmount seems to only have one
call site so perhaps these can be merged.

[NO NEW TESTS NEEDED]

Signed-off-by: Doug Rabson <dfr@rabson.org>

libpod/volume_internal_linux.go

@@ -0,0 +1,9 @@
+package libpod
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+func detachUnmount(mountPoint string) error {
+	return unix.Unmount(mountPoint, unix.MNT_DETACH)
+}