ldb_mdb: Use mdb_env_get_fd() to get the FD for fstat() and FD_CLOEXEC

ldb_mdb: Use mdb_env_get_fd() to get the FD for fstat() and FD_CLOEXEC

This ensures we leave the FD behind if we exec() in a child process.

This deliberatly the same as TDB, as we want the same behaviour as
we have come to expect with that backend.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>