s3: Add extid to the dev/inode pair

s3: Add extid to the dev/inode pair

This extends the file_id struct to add an additional generic uint64_t
field: extid.  For backwards compatibility with dev/inodes stored in
xattr_tdbs and acl_tdbs, the ext id is ignored for these databases.
This patch should cause no functional change on systems that don't use
SMB_VFS_FILE_ID_CREATE to set the extid.

Existing code that uses the smb_share_mode library will need to be
updated to be compatibile with the new extid.