md/md-bitmap: fix stats collection for external bitmaps

The bitmap_get_stats() function incorrectly returns -ENOENT for external
bitmaps.

Remove the external bitmap check as the statistics should be available
regardless of bitmap storage location.

Return -EINVAL only for invalid bitmap with no storage (neither in
superblock nor in external file).

Note: "bitmap_info.external" here refers to a bitmap stored in a separate
file (bitmap_file), not to external metadata.

Fixes: 8d28d0dd ("md/md-bitmap: Synchronize bitmap_get_stats() with bitmap lifetime")
Signed-off-by: Zheng Qixing <zhengqixing@huawei.com>
Link: https://lore.kernel.org/linux-raid/20250403015322.2873369-1-zhengqixing@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>