mds: fix bad import_state check on handle_export_discover
mds: fix bad import_state check on handle_export_discover

This populates import_state[] with an bad value and leads to crashes like

mds/Migrator.h: In function 'static const char* Migrator::get_import_statename(int)', in thread '0x7f5ea8c97710'
mds/Migrator.h: 112: FAILED assert(0)
 ceph version 0.25-670-g85bd67e (commit:85bd67e0ab58876ad807b44ab2154e84b90a4f30)
 1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x53) [0xa53ad6]
 2: (Migrator::get_import_statename(int)+0x68) [0x91ea0f]
 3: (Migrator::show_importing()+0x174) [0x90f640]

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph