Use SMB_QUERY_FS_SIZE_INFO request to populate statfs structure.
When server doesn't support this request, try to use SMB_INFO_ALLOCATION.
And use SMB_COM_QUERY_INFORMATION_DISK request as fallback.
MFC r264600:
Remove redundant unlock.
This code was removed from the opensolaris and darwin's
netsmb implementations, in DfBSD it also has been disabled.