FAT: add 'errors' mount option

On severe errors FAT remounts itself in read-only mode. Allow to
specify FAT fs desired behavior through 'errors' mount option:
panic, continue or remount read-only.

`mount -t [fat|vfat] -o errors=[panic,remount-ro,continue] \
	<bdev> <mount point>`

This is analog to ext2 fs 'errors' mount option.

Signed-off-by: Denis Karpov <ext-denis.2.karpov@nokia.com>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

fs/fat/namei_msdos.c

@@ -608,7 +608,7 @@ error_inode:
 		sinfo.bh = NULL;
 	}
 	if (corrupt < 0) {
-		fat_fs_panic(new_dir->i_sb,
+		fat_fs_error(new_dir->i_sb,
 			     "%s: Filesystem corrupted (i_pos %lld)",
 			     __func__, sinfo.i_pos);
 	}