use linux/export.h rather than asm-generic/export.h

asm-generic/export.h is a wrapper for linux/export.h, with explicit request
to use linux/export.h directly.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20231214191922.GQ1674809@ZenIV
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>