Bugfix: Directory sync errors for repositories accessed via SMB

On Linux and macOS, accessing a repository via a SMB/CIFS mount resulted in
restic failing to save the lock file, yielding the following errors:

Save(<lock/071fe833f0>) returned error, retrying after 552.330144ms: sync /repo/locks: no such file or directory
Save(<lock/bf789d7343>) returned error, retrying after 552.330144ms: sync /repo/locks: invalid argument

This has now been fixed by ignoring the relevant error codes.

https://github.com/restic/restic/issues/3720
https://github.com/restic/restic/issues/3751
https://github.com/restic/restic/pull/3752
