lowmem-monitor: Check return value of eventfd_write and remove useless code

lowmem-monitor: Check return value of eventfd_write and remove useless code

Check return value of eventfd_write and remove eventfd_read function. This is
because if eventfd_write is not correctly working (return value is -1),
then monitor pthread will not be exited. In addition, eventfd_read is used to
read dummy value, so psi monitor will be exited correctly regardless of eventfd_read.

It solves problems reported by Coverity with cid = 1704480 and cid = 1704688.

Change-Id: Ic753aaf3b96c95d7f1e2e629c165800728b92289
Signed-off-by: Unsung Lee <unsung.lee@samsung.com>