Retry unmounts in ext4 encryption

Bug: 18151196
Change-Id: I52ca23b2ce3adcff44bd003d4a12243a0bd6ac34
diff --git a/cryptfs.c b/cryptfs.c
index 13ccd6c..5598671 100644
--- a/cryptfs.c
+++ b/cryptfs.c
@@ -1446,7 +1446,7 @@
     return encrypt_master_key(passwd, salt, key_buf, master_key, crypt_ftr);
 }
 
-static int wait_and_unmount(char *mountpoint, bool kill)
+int wait_and_unmount(const char *mountpoint, bool kill)
 {
     int i, err, rc;
 #define WAIT_UNMOUNT_COUNT 20