2493 change efi_rescan() to wait longer

Change efi_rescan() to loop 10 times instead of 5 on EBUSY and
to sleep at the end of each loop. This helps with some instances
where the kernel does not reload the partition table fast enough
for ZFS to detect.

Signed-off-by: Andrew Hamilton <ahamilto@tjhsst.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2493