ZTS: Fix parsing of zpool status in checksum test

ZTS: Fix parsing of zpool status in checksum test

filetest_001_pos consumes the output using read -r, assigning each
field to a variable. The problem comes when a vdev is marked degraded,
which appends extra fields to the line. This causes the trailing text
to be treated as part of the `cksum` variable. Using awk instead of
read -r allows us to extract the checksum error count from the output
whether the vdev is degraded or not.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Wren Kennedy <john.kennedy@delphix.com>
Closes #8136