linux(4): Fix listxattr for the case when the size is 0

If size is specified as zero, these calls return the current size
of the list of extended attribute names (and leave list unchanged).

Approved by: re (gjb)
Tested by: zirias
MFC after: 1 week

(cherry picked from commit 18d1c86788f66f42c4e096142f4f8d168f68732c)
(cherry picked from commit 53b3e15d732ff6ff0df7870107d45a371a376e73)