zpool: speed up importing large pools (#11469)

The ZFS_IOC_POOL_TRYIMPORT ioctl returns an nvlist from the kernel to a
preallocated buffer in  userland.  Userland must guess how large the
buffer should be.  If it undersizes it, it must reallocate and try
again.  That can cost a lot of time for large pools.

OpenZFS commit 28b40c8a6e3 set the guess at "zc.zc_nvlist_conf_size * 4"
without explanation.  On my system, that is too small.  From experiment,
x 32 is a better multiplier.  But I don't know how to calculate it
theoretically.

Sponsored by:	Axcient
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alek Pinchuk <[email protected]>
Signed-off-by:	Alan Somers <[email protected]>
Closes #11469 

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