snapuserd: Remove IOSQE_ASYNC flag

IOSQE_ASYNC will forcefully put the task in async mode which is
not required.

Bug: 425765067
Test: OTA on Pixel
Change-Id: I59d2eb35f6a4f071e122693e1b9e20afe996349e
Signed-off-by: Akilesh Kailash <akailash@google.com>