Fix errno check based on EINTR in pg_flush_data()

Fix errno check based on EINTR in pg_flush_data()

Upon a failure of sync_file_range(), EINTR was checked based on the
returned result of the routine rather than its errno.  sync_file_range()
returns -1 on failure, making the check a no-op, invalidating the retry
attempt in this case.

Oversight in 0d369ac65004.

Author: DaeMyung Kang <charsyam@gmail.com>
Discussion: https://postgr.es/m/20260429151811.1810874-1-charsyam@gmail.com
Backpatch-through: 16