mirror: Minor refactoring

mirror: Minor refactoring

Commit 5791ba52 (v9.2) pre-initialized ret in mirror_dirty_init to
silence a false positive compiler warning, even though in all code
paths where ret is used, it was guaranteed to be reassigned
beforehand.  But since the function returns -errno, and -1 is not
always the right errno, it's better to initialize to -EIO.

An upcoming patch wants to track two bitmaps in
do_sync_target_write(); this will be easier if the current variables
related to the dirty bitmap are renamed.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250509204341.3553601-21-eblake@redhat.com>