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/