Use kmem_vasprintf() in log_internal()

An attempt to debug zfsonlinux/zfs#2781 revealed that this code could be
simplified by using kmem_asprintf(). It is not clear that switching to
kmem_asprintf() addresses zfsonlinux/zfs#2781. However, switching to
kmem_asprintf() is cleanup that simplifies debugging such that it would
become clear that this is a bug in glibc should the issue persist.

Switching to kmem_asprintf() has the side effect of making
log_internal() allocate memory with KM_SLEEP instead of KM_PUSHPAGE.
That allocation originally used KM_SLEEP until it was switched to
KM_PUSHPAGE in b8d06fca089fae4680c3a552fc55c512bfb02202 as part of an
attempt to prevent deadlocks on swap. A cursory review suggests that
log_internal() is only used outside of DMU transactions, the sync task
and the quiesce task. It follows that the original rationale for
switching this particular allocation from KM_SLEEP to KM_PUSHPAGE was
invalid. Switching back to KM_SLEEP is therefore not only safe, but
desireable to avoid unnecessary use of the kernel's emergency memory
pools under low memory situations.

This also implicitly reverts 222b94805903dfa6879565ab9b1c8e3b0d70cbdf by
using `strfree()` instead of `kmem_free()`. This is possible because
kmem_vasprintf() is implicitly mapped to kmalloc() by the SPL, which
avoids creating a false positive in the kmem leak detection.

Signed-off-by: Richard Yao <ryao@gentoo.org>