Fix reservation test cases for large disks

Convert explicit typeset -i and typeset -l declarations to
typeset in order to prevent 32-bit overflow from occurs with
disks >2G.

TEST_ZFSTESTS_DISKSIZE=4G

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5715
Closes #5714