smbd: avoid calling vfs_file_id_from_sbuf() if statinfo is not valid

When we're about to create a file, the stat info will be all zero, so
vfs_file_id_from_sbuf() would return a bogus file_id. This is normally not a
problem, as open_file() itself also calls vfs_file_id_from_sbuf() after having
created the file.

This is however a problem when using the VFS module fileid, as that is doing
caching of /etc/mtab and failing to find smb_fname->st.st_ex_dev (all zero in
this case when creating a new file) in the mtab cache will trigger a mtab reload
which can be *very* expensive.

Copying many small files to a Samba server in this situation will result in
abysimal performance.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14237

Pair-Programmed-With: Jeremy Allison <jra@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Jan 17 22:38:14 UTC 2020 on sn-devel-184
