Assert alignment in umem_alloc_aligned

Valgrind suggests that the address we are returning is not properly
aligned, so lets add an assertion.

==87740==  Address 0x1012a22a is 554 bytes inside a block of size 4,096
alloc'd
==87740==    at 0x4C2BBA0: memalign (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==87740==    by 0x4C2BCC7: posix_memalign (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==87740==    by 0x52FA845: zio_buf_alloc (umem.h:101)
==87740==    by 0x52F6226: zil_alloc_lwb (zil.c:463)
==87740==    by 0x52F8559: zil_commit (zil.c:566)
==87740==    by 0x40611D: ztest_freeze (ztest.c:5909)
==87740==    by 0x4066A7: ztest_init (ztest.c:6048)
==87740==    by 0x407AF4: main (ztest.c:6226)

Signed-off-by: Richard Yao <ryao@gentoo.org>