Some of modpost's warnings are fatal, and some are not.  Adopt the
compiler distinction between errors and warnings by calling merror()
for fatal diagnostics and warn() for non-fatal ones.
merror() was used as replacemtn for error() to avoid clash with glibc
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>