mds: Setting pool on a file requires latest osdmap

If we create a file, then create a pool, then try to
set the pool on the file with the vxattr, no mds operation
triggers a request for the latest osdmap, so we get back
EINVAL.  So if we fail to find the pool initially, fall back
to requesting a new osdmap and trying the vxattr again.  We
don't wait for a new map if we don't get anything newer than
what we already have.

Signed-off-by: Sam Lang <sam.lang@inktank.com>