Handle partial reads in zfs_read

Currently, dmu_read_uio_dnode can read 64K of a requested 1M in one
loop, get EFAULT back from zfs_uiomove() (because the iovec only holds
64k), and return EFAULT, which turns into EAGAIN on the way out. EAGAIN
gets interpreted as "I didn't read anything", the caller tries again
without consuming the 64k we already read, and we're stuck.

This apparently works on newer kernels because the caller which breaks
on older Linux kernels by happily passing along a 1M read request and a
64k iovec just requests 64k at a time.

With this, we now won't return EFAULT if we got a partial read.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rich Ercolani <[email protected]>
Closes #12370 
Closes #12509
Closes #12516 

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