gdb/linux-nat: xfer_memory_partial return E_IO on error

gdb/linux-nat: xfer_memory_partial return E_IO on error

When accessing /proc/PID/mem, if pread64/pwrite64/read/write encounters
an error and return -1, linux_proc_xfer_memory_partial return
TARGET_XFER_EOF.

I think it should return TARGET_XFER_E_IO in this case.  TARGET_XFER_EOF
is returned when pread64/pwrite64/read/frite returns 0, which indicates
that the address space is gone and the whole process has exited or
execed.

This patch makes this change.

Regression tested on x86_64-linux-gnu.

Change-Id: I6030412459663b8d7933483fdda22a6c2c5d7221