ext4: add mext_check_validity() to do basic check

ext4: add mext_check_validity() to do basic check

Currently, the basic validation checks during the move extent operation
are scattered across __ext4_ioctl() and ext4_move_extents(), which makes
the code somewhat disorganized. Introduce a new helper,
mext_check_validity(), to handle these checks. This change involves only
code relocation without any logical modifications.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Message-ID: <20251013015128.499308-7-yi.zhang@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>