Update system/xattrs_linux.go

- Add Llistxattr() support
- Additionally cleanup Lgetxattr() and implement it properly
  in accordance with getxattr() syscall behavior.

Signed-off-by: Harshavardhana <fharshav@redhat.com>
Maybe combine these 3 tests into one, so we can check that getxattr and listxattr get back the values set here?
Also, setting a value > 128 bytes would help check the scenario of re-sizing the buffer.