mds: stay in SCAN state in file_eval

If we are in the SCAN state, stay there until the recovery finishes.  Do
not jump to another state from file_eval().

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 0071b8e75bd3f5a09cc46e2225a018f6d1ef0680)