mon/MDSMonitor: don't reset incarnation when creating newfs

Fixes: #6279

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
(cherry picked from commit 40613b700b87b495d67793101ae05d6ba58b2a9a)
Reviewed-by: Greg Farnum <greg@inktank.com>
mds: return -EAGAIN if standby replay falls behind

standby replay may fall behind and get -ENOENT when reading the
journal. return -EAGAIN in this case, it makes the MDS respawn itself.

fixes: #5458

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit d2cb2bf6bac83ac6db9df6cb876317d30e7493cc)
Reviewed-by: Greg Farnum <greg@inktank.com>