linux: libzfs: simplify module-loaded check

The short-path is now one access() call,
we always modprobe zfs (ZFS_MODULE_LOADING which doesn't use the libzfs
boolean parsing is gone),
and we use a simple inotify IN_CREATE loop with a timerfd timeout
rather than 10ms kernel-style polling

There's one substantial difference: ZFS_MODULE_TIMEOUT=-1
now means "never give up", rather than "wait 10 minutes"

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13330