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
Illumos 5034 - ARC's buf_hash_table is too small

5034 ARC's buf_hash_table is too small

Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Reviewed by: Richard Elling <richard.elling@gmail.com>
Approved by: Gordon Ross <gwr@nexenta.com>

References:
  https://www.illumos.org/issues/5034
  https://github.com/illumos/illumos-gate/commit/63e911b

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2615