QStorageInfo: fix matching of mountpoints to sibling directories

The path "/usrfoo" starts with "/usr", so if you tried to get
QStorageInfo("/usrfoo") when "/usr" is a mount point, you'd get the
wrong filesystem.

[ChangeLog][QtCore][QStorageInfo] Fixed a bug that caused QStorageInfo
to report information for the wrong filesystem if there is a mounted
filesystem at a path that is a prefix of the requested path (e.g., it
would report "/usr" filesystem for "/usrfoo").

Task-number: QTBUG-49498
Change-Id: I3e15a26e0e424169ac2bffff1417b7a27cd0132d
Reviewed-by: Jake Petroules <jake.petroules@qt.io>