s3: Fix trans2 path to use case-insensitive stat optimization

s3: Fix trans2 path to use case-insensitive stat optimization

Often times before creating a file, a client will first query to see
if it already exists.  Since some systems have a case-insensitive stat
that is called from unix_convert, we can definitively return
STATUS_NO_SUCH_FILE to the client without scanning the whole
directory.

This code path is taken from trans2querypathinfo, but trans2findfirst
still does a full directory scan even though the get_real_filename
(the case-insensitive stat vfs call) can prevent this.

This patch adds the get_real_filename call to the trans2find* path,
and also changes the vfs_default behavior for
SMB_VFS_GET_REAL_FILENAME.  Previously, in the absence of a
get_real_filename implementation, we would fallback to the full
directory scan.  The default behavior now returns -1 and sets errno to
EOPNOTSUPP.  This allows SMB_VFS_GET_REALFILENAME to be called from
trans2* and unix_convert.