fat: use fat_fs_error() instead of BUG_ON() in __fat_get_block()

If file size and FAT cluster chain is not matched (corrupted image), we
can hit BUG_ON(!phys) in __fat_get_block().

So, use fat_fs_error() instead.

[hirofumi@mail.parknet.co.jp: fix printk warning]
  Link: http://lkml.kernel.org/r/87po12aq5p.fsf@mail.parknet.co.jp
Link: http://lkml.kernel.org/r/874lilcu67.fsf@mail.parknet.co.jp


Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Tested-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>