pool: Fix xrootd vector read limits

Fixes a bug that caused problems with ROOT 6: dCache provided
too large limits for the readv_ior_max and readv_iov_max
properties.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8461
Require-notes: ues
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7313/
(cherry picked from commit 92b36682cf3dd4aab14dab64ded6bf40e5660f4d)
Merge pull request #854 from gbehrmann/fix/2.7/rb7313

pool: Fix xrootd vector read limits