[acl_loader]:Fix show mirror_session error

[JaredLius]

Signed-off-by: Jared.Liu [email protected]

- What I did
when execute command "show mirror_session", the status display error.
admin@sonic:~$ show mirror_session
Name Status SRC IP DST IP GRE DSCP TTL Queue

---

everflow0 error 14.14.14.149 192.192.2.2 0x6558 50 64
there is some code error in acl_loader/main.py.
- How I did it
when get mirror session information from state_db, use MIRROR_SESSION_TABLE instead of MIRROR_SESSION
- How to verify it
After fix, the status display correct.
admin@sonic:~$ show mirror_session
Name Status SRC IP DST IP GRE DSCP TTL Queue Policer

---

everflow0 active 14.14.14.149 192.192.2.2 0x6558 50 64
- Previous command output (if the output of a command-line utility has changed)

- New command output (if the output of a command-line utility has changed)

-->

[stcheng]

Thanks for the fixing!