fat: fix error message for bogus number of directory entries

"bogus directory-entries per block" was reported for what was instead
bogus number of directory entries. The message also mismatched the
argument passed to printk(), which was sbi->dir_entries.

Fix this by replacing the message with "bogus number of directory
entries". printk() argument was kept unchanged.

Signed-off-by: Zheng Lv <lv.zheng.2015@gmail.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>