mds: fix issues with use-after-free in C_Flush_Journal

mds: fix issues with use-after-free in C_Flush_Journal

Resolved use-after-free issue of ESubtreeMap. The subtreemap event gets
destroyed after it is submitted to the log.

MDLog::submit_event() now returns a sequence number of the submitted event.

Fixes: https://tracker.ceph.com/issues/69953
Signed-off-by: Milind Changire <mchangir@redhat.com>
(cherry picked from commit e1ab8eb78b17b2c0ca606e173f3015c909092700)

Conflicts:
	src/mds/MDLog.h
	src/mds/MDSRank.cc
	- resolved conflicts due to main and squid branch divergence