Fix unchecked return values and unused return values

Coverity complained about unchecked return values and unused values that
turned out to be unused return values.

Different approaches were used to handle the different cases of
unchecked return values:

• cmd/zdb/zdb.c: VERIFY0 was used in one place since the existing code had no error handling. An error message was printed in another to match the rest of the code.

• cmd/zed/agents/zfs_retire.c: We dismiss the return value with (void) because the value is expected to be potentially unset.

• cmd/zpool_influxdb/zpool_influxdb.c: We dismiss the return value with (void) because the values are expected to be potentially unset.

• cmd/ztest.c: VERIFY0 was used since we want failures if something goes wrong in ztest.

• module/zfs/dsl_dir.c: We dismiss the return value with (void) because there is no guarantee that the zap entry will always be there. For example, old pools imported readonly would not have it and we do not want to fail here because of that.

• module/zfs/zfs_fm.c: fnvlist_add_*() was used since the allocations sleep and thus can never fail.

• module/zfs/zvol.c: We dismiss the return value with (void) because we do not need it. This matches what is already done in the analogous zfs_replay_write2().

• tests/zfs-tests/cmd/draid.c: We suppress one return value with (void) since the code handles errors already. The other return value is handled by switching to fnvlist_lookup_uint8_array().

• tests/zfs-tests/cmd/file/file_fadvise.c: We add error handling.

• tests/zfs-tests/cmd/mmap_sync.c: We add error handling for munmap, but ignore failures on remove() with (void) since it is expected to be able to fail.

• tests/zfs-tests/cmd/mmapwrite.c: We add error handling.

As for unused return values, they were all in places where there was
error handling, so logic was added to handle the return values.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13920