CVE-2019-10197: smbd: split change_to_user_impersonate() out of change_to_user_internal()

CVE-2019-10197: smbd: split change_to_user_impersonate() out of change_to_user_internal()

This makes sure we always call chdir_current_service() even
when we still impersonated the user. Which is important
in order to run the SMB* request within the correct working directory
and only if the user has permissions to enter that directory.

It makes sure we always update conn->lastused_count
in chdir_current_service() for each request.

Note that vfs_ChDir() (called from chdir_current_service())
maintains its own cache and avoids calling SMB_VFS_CHDIR()
if possible.

It means we still avoid syscalls if we get a multiple requests
for the same session/tcon tuple.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14035

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(master): Tue Sep  3 09:27:22 UTC 2019 on sn-devel-184