fs/ntfs3: Avoid UBSAN error on true_sectors_per_clst()

syzbot reported UBSAN error as below:

[   76.901829][ T6677] ================================================================================
[   76.903908][ T6677] UBSAN: shift-out-of-bounds in fs/ntfs3/super.c:675:13
[   76.905363][ T6677] shift exponent -247 is negative

This patch avoid this error.

Link: https://syzkaller.appspot.com/bug?id=b0299c09a14aababf0f1c862dd4ebc8ab9eb0179


Fixes: a3b77434 (fs/ntfs3: validate BOOT sectors_per_clusters)
Cc: Author: Randy Dunlap <rdunlap@infradead.org>
Reported-by:  <syzbot+35b87c668935bb55e666@syzkaller.appspotmail.com>
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>