zero-filling fuse_session structure if session is not yet initialized

[qkrorlqr]

to fix msan issues like this:
https://github-actions-s3.website.nemax.nebius.cloud/ydb-platform/nbs/Nightly-build-(msan)/12404651235/1/nebius-x86-64-msan/test_data/actions-runner/_work/nbs/nbs/cloud/filestore/tests/fio_index/qemu-kikimr-multishard-nemesis-test/test-results/py3test/chunk4/testing_out_stuff/filestore-vhost.err

==465640==WARNING: MemorySanitizer: use-of-uninitialized-value

    ... куча наших логов + gdb bt ... 

    #0 0x3987aa48 in process_request /actions-runner/_work/nbs/nbs/cloud/filestore/libs/vfs_fuse/vhost/fuse_virtio.c
    #1 0x3987aa48 in virtio_session_loop /actions-runner/_work/nbs/nbs/cloud/filestore/libs/vfs_fuse/vhost/fuse_virtio.c:426:19
    #2 0x397a64f6  (/home/github/.ya/build/build_root/snjo/002a89/cloud/filestore/apps/vhost/filestore-vhost+0x397a64f6) (BuildId: f8e58c8c86d9a034d54ab2fa48f434506868888c)
    #3 0x101d3ea2 in (anonymous namespace)::TPosixThread::ThreadProxy(void*) /actions-runner/_work/nbs/nbs/util/system/thread.cpp:244:20
    #4 0x7f7737968ac2  (/lib/x86_64-linux-gnu/libc.so.6+0x94ac2) (BuildId: 490fef8403240c91833978d494d39e537409b92e)
    #5 0x7f77379fa84f  (/lib/x86_64-linux-gnu/libc.so.6+0x12684f) (BuildId: 490fef8403240c91833978d494d39e537409b92e)

it's hard to say how exactly we run into such a situation
most probably se->bufsize actually is initialized but in a way that's not seen by msan
it's surely initialized in virtiofsd/fuse_lowlevel.c::do_init which, together with fuse_session_setparams, should cover all code paths

[github-actions]

Note

This is an automated comment that will be appended during run.

🔴 linux-x86_64-release-msan: some tests FAILED for commit d2be771.

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
1630	1480	0	150	0	0

[github-actions]

Note

This is an automated comment that will be appended during run.

🔴 linux-x86_64-relwithdebinfo: some tests FAILED for commit efeeb45.

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
1654	1503	0	149	2	0

[github-actions]

Note

This is an automated comment that will be appended during run.

🔴 linux-x86_64-relwithdebinfo: some tests FAILED for commit efeeb45.

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
1654	1503	0	149	2	0

---

🔴 linux-x86_64-release-msan: some tests FAILED for commit efeeb45.

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
1631	1487	0	142	2	0