xattr: buffer full check for freebsd

freebsd 10.2:

it does not give rc < 0 and errno == ERANGE if the buffer was too small,
like linux or mac OS X does.

rv == buffer len might be a signal of truncation.
rv > buffer len would be even worse

not sure if some implementation returns the total length of the data,
not just the amount put into the buffer.

but as we use the returned length to "truncate" the buffer, we better
make sure it is not longer than the buffer.

also: freebsd listxattr memoryview len bugfix

borg/xattr.py

@@ -128,6 +128,12 @@ def _check(rv, path=None, detect_buffer_too_small=False):
             if isinstance(path, int):
                 path = '<FD %d>' % path
             raise OSError(e, msg, path)
+    if detect_buffer_too_small and rv >= len(get_buffer()):
+        # freebsd does not error with ERANGE if the buffer is too small,
+        # it just fills the buffer, truncates and returns.
+        # so, we play sure and just assume that result is truncated if
+        # it happens to be a full buffer.
+        raise BufferTooSmallError
     return rv
 
 
@@ -323,7 +329,7 @@ elif sys.platform.startswith('freebsd'):  # pragma: freebsd only
         if n == 0:
             return []
         names = []
-        mv = memoryview(buf)
+        mv = memoryview(buf)[:n]
         while mv:
             length = mv[0]
             names.append(os.fsdecode(bytes(mv[1:1 + length])))