srm: Fix regression in listing busy SURLs

Addresses an ARC compatibility issue.

ARC issues an srmLs against a SURL after uploading a file, but
before calling srmPutDone. It does this to verify file size and
checksum before releasing the SURL to other clients.

Ever since we implemented SRM_FILE_BUSY, this logic no longer works,
as we skip the meta data lookup for busy files. This issue is
addressed by this patch.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6147/