Fix input-linux reading from device

Fix input-linux reading from device

The evdev devices in input-linux.c are read in blocks of one whole
event. If there are not enough bytes available, they are discarded,
instead of being kept for the next read operation. This results in
lost events, of even non-working devices.

This patch keeps track of the number of bytes to be read to fill up
a whole event, and then handle it.

Changes from v1 to v2:
- Fix: Calculate offset on each iteration

Changes from v2 to v3:
- Fix coding style
- Store offset instead of bytes to be read

Signed-off-by: Javier Celaya <jcelaya@gmail.com>
Message-id: 20170327182624.2914-1-jcelaya@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>