some enhancements

[liulanzheng]

What this PR does / why we need it:

• ensure that everlaybd-seal only applies to overlaybd writable layer
• remove SupportReadWriteMode label after seal
• always try unmount mountpoint even for rwdev, unmounting a not mounted dir has no side effects for mount.UnmountAll

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[yuchen0cc]

LGTM