Fix smbfs to not zero out statfs f_flags field. Previously, this
made getmntinfo() return empty flags for smbfs filesystems when
called with MNT_WAIT. It's not visible with mount(8), since it uses
MNT_NOWAIT, but broke autounmount(8) operation.

PR: 195161
Differential Revision: https://reviews.freebsd.org/D1194
Reviewed by: kib@
MFC after: 1 month
Sponsored by: The FreeBSD Foundation