mds/Locker: use ceph_abort_msg() instead of ceph_assert()

mds/Locker: use ceph_abort_msg() instead of ceph_assert()

This ceph_assert() always fails, but depending on the configuration
value `ceph_assert_supresssions`, execution may continue, but the
`dir` variable is left uninitialized.  This leads to a compiler
warning:

 /home/jenkins-build/build/workspace/ceph-api/src/mds/Locker.cc:451:22: error: variable 'dir' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]

clang then suggests to nullptr-initialize the variable:

 /home/jenkins-build/build/workspace/ceph-api/src/mds/Locker.cc:447:11: note: initialize the variable 'dir' to silence this warning
   447 |         CDir *dir;
       |                  ^
       |                   = nullptr

This, however, is a very bad idea because all this does is suppress
the warning; it still crashes the process.

Since there's no recovery from this problem, let's switch to
ceph_abort_msg() which is [[noreturn]] and the compiler can deduce
that `dir` is always initialized when it's used.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>