Add handling for kernel 4.7's CONFIG_TRIM_UNUSED_KSYMS

Add handling for kernel 4.7's CONFIG_TRIM_UNUSED_KSYMS

Kernel 4.7 added the option to trim the unused exported symbols. In
my testing this showed to be problematic since the PDE_DATA function
was considered unused and as such was trimmed. This in turn caused the
respective test during spl's configure stage to falsely detect that
PDE_DATA is not defined, which in turn caused build failures later.

Handle this situation by adding detection whether CONFIG_TRIM_UNUSED_KSYMS
is enabled and refuse to build against a kernel which has it enabled

Signed-off-by: Nikolay Borisov <n.borisov.lkml@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #565