vfs_lxip: propagate data file write error

All errors (as in the legacy version) are now propagated as WRITE_ERR_IO
(see Lxip_vfs_file_handle::write), which ultimately will lead to EPIPE
in libc's socket_fs. This also counts for EAGAIN leading to the fact
that partial writes are not supported for blocking sockets in libc, also
libc will not try a second time in case not all data has been written.

issue #5165

repos/dde_linux/src/lib/vfs/lxip/vfs.cc

@@ -475,7 +475,12 @@ class Vfs::Lxip_data_file final : public Vfs::Lxip_file
 
 			_write_err = genode_socket_sendmsg(&_sock, msg_send.header(), &bytes_sent);
 
-			return bytes_sent;
+			/* TODO: propagate EAGAIN
+			if (_write_err == GENODE_EAGAIN)
+				throw Would_block();
+			*/
+
+			return _write_err == GENODE_ENONE ? bytes_sent : -1;
 		}
 
 		long read(Lxip_vfs_file_handle &,