ext2fs: don't retry discard/zeroout repeatedly

ext2fs: don't retry discard/zeroout repeatedly

Call safe_getenv(UNIX_IO_NOZEROOUT) once when the device is
opened and set CHANNEL_FLAG_NOZEROOUT if present instead of
getting uid/euid/getenv every time unix_zeroout() is called.

For unix_discard() and unix_zeroout() don't continue to call
them if the block device doesn't support these operations.

Change-Id: I3b3b79cf49a6f1cf9b254eb32103f174bacdfbd2
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/tools/e2fsprogs/+/51294
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>