Remove use of ngroups return value to size the buffer

As mentioned in the PR comments, we may want to dynamically detect if the runtime `libc` appears to be setting `ngroups` to the required buffer size, rather than hardcoding based on target OS.  That may require a bit more conversation, and I think it's best to not tie up the buffer overrun fix in the meantime.

src/unistd.rs

@@ -1567,16 +1567,8 @@ pub fn getgrouplist(user: &CStr, group: Gid) -> Result<Vec<Gid>> {
             // BSD systems will still fill the groups buffer with as many
             // groups as possible, but Linux manpages do not mention this
             // behavior.
-            // Linux stores the number of groups found in ngroups.  We can
-            // use that to resize the buffer exactly.
-            cfg_if! {
-                if #[cfg(target_os = "linux")] {
-                    groups.reserve_exact(ngroups as usize);
-                } else {
-                    reserve_double_buffer_size(&mut groups, ngroups_max as usize)
-                        .map_err(|_| Errno::EINVAL)?;       
-                }
-            }            
+            reserve_double_buffer_size(&mut groups, ngroups_max as usize)
+                .map_err(|_| Errno::EINVAL)?;
         }
     }
 }