fs_mgr_overlayfs: Polish fs_mgr_overlayfs_mount_fstab_entry()

* Add logs.
* Append "override_creds=off" overlayfs mount flag only if
  fs_mgr_overlayfs_valid() returns kOverrideCredsRequired.
  Pre-4.6 kernels or kernels without the override_creds patch don't
  need or don't recognize the override_creds mount flag.
  (Background: I832c8ca3fce0269bdef4ce988541adb7ba9662ed)
* mkdir(mount_point) before mount() to ensure the mount point exists.
  This could happen if the mount point is in a tmpfs, such as /mnt.

Bug: 186342252
Test: Boot to normal with overlayfs mount entries in first stage fstab
Change-Id: I1a05696346610d7fd61de6d25c379520fd58ca9b