fs: fs_unmount will return -ENOTSUP if not implemented by driver

The commit changes error handling by fs_unmount; the function will
return -EINVAL if mount point, described by mp, is not mounted or
-ENNOTSUP when unmounting is not supported by the driver; in the second
case it will also log error.

Additionally to the above changes, checks for correct mnt_path and
mnt_path, within fs_unmount, have been removed as they are not needed;
only the fs_mount_t->fs pointer is needed to decide whether system is
mounted or not.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>