rpcrdma: Use XA_FLAGS_ALLOC instead of XA_FLAGS_ALLOC1

Nit: The built-in xa_limit_32b range starts at 0, but
XA_FLAGS_ALLOC1 configures the xarray's allocator to start at 1.
Adopt the more conventional XA_FLAGS_ALLOC because there's no
mechanical reason to skip 0.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>