fbo: fix pread/pwrite return handling
fbo: fix pread/pwrite return handling

pread/pwrite return -1 on failure so we need to use a ssize_t
instead of a size_t.

Signed-off-by: Mike Christie <39daeb1549a929782e38aa87c4e3da7f3844b09a@redhat.com>

open-iscsi/tcmu-runner,agrover/tcmu-runner,open-iscsi/tcmu-runner