dynamic_debug: add pr_errs before -EINVALs

Ma noted that dynamic-debug is silent about many query errors, so add
pr_err()s to explain those errors, and tweak a few others.  Also parse
flags 1st, so that match-spec errs are slightly clearer.

CC: Jianpeng Ma <majianpeng@gmail.com>
CC: Joe Perches <joe@perches.com>
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>