Fix zpool_read_all_labels when vfs.aio.enable_unsafe=0

Previously, zpool_read_all_labels was trying to do 256KB reads, which are
greater than the default MAXPHYS and therefore must go through the slow,
unsafe AIO path. Shrink these reads to 112KB so they can use the safe, fast
AIO path instead.

MFC after: 1 week
X-MFC-With: 324568
Sponsored by: Spectra Logic Corp