Excessively large stack frames detected.

Excessively large stack frames detected.
The 2.6.30 kernel build systems sets -Wframe-larger-than=2048 which causes
a warning to be generated when an individual stack frame exceeds 2048.
This caught the spa_history_log() and dmu_objset_snapshot() functions
which declared a data structure on the stack which contained a char
array of MAXPATHLEN.  This in defined to be 4096 in the linux kernel
and I imagine it is quite large under Solaris as well.  Regardless, the
offending data structures were moved to the heap to correctly keep the
stack depth to a minimum.  We might consider setting this value even
lower to catch additional offenders because we are expecting deep stacks.