mon: EBUSY instead of EAGAIN when pgs creating

In 69321bf, EAGAIN changed behaviour to block indefinitely
rather than returning to user.  Change the return for
`osd pool set` operations that are blocked by creating PGs
to return EBUSY instead of EAGAIN, so that they are excepted
from this blocking behaviour.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1677 from ceph/wip-poolset-noblock

mon: Don't block on EAGAIN from `osd pool set`

Reviewed-by: Sage Weil <sage@inktank.com>