[Virtio-fs] [virtiofsd] MR opened: [Draft] Keep DAC_READ_SEARCH on setxattr with posix acl

[virtiofs-bot at sinrega.org]

If posix acl is enabled we change the effective uid, if it's changed
to a non-zero uid hen the effective set is cleared from all
capabilities. We need to keep `DAC_READ_SEARCH` if file handles is
enabled. This fixes the xfs test `generic/375`.

closes #56
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/140