kentry: Verify kentry state in instrumentation_begin/end()

Calling instrumentation_begin() and instrumentation_end() when kentry
thinks the CPU is in user mode is an error.  Verify the kentry state when
instrumentation_begin/end() are called.

Add _nocheck() variants to skip verification to avoid WARN() generating
extra kentry warnings.

Signed-off-by: Andy Lutomirski <luto@kernel.org>