Avoid reading past the end of the disk in zfsboot.c and biosdisk.c

The GELI boot code rounds reads up to 4k, since the encrypted sectors are
4k, and must be decrypted as a unit. With oddball sized disks (almost
always virtual), this can lead to reading past the end of the disk.

Reviewed by: imp, tsoome
Sponsored by: Klara Systems
Differential Revision: https://reviews.freebsd.org/D15844