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.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/1683695929-26972-1-git-send-email-adilger@dilger.ca
Signed-off-by: Theodore Ts'o <tytso@mit.edu>