fsmap.h: add linux/fsmap.h to UAPI compile-test coverage

linux/fsmap.h is currently excluded from the UAPI compile-test because
of the error like follows:

    HDRTEST usr/include/linux/fsmap.h
  In file included from <command-line>:
  ./usr/include/linux/fsmap.h:72:19: error: unknown type name ‘size_t’
     72 | static __inline__ size_t
        |                   ^~~~~~

The error can be fixed by replacing size_t with __kernel_size_t.

Then, remove the no-header-test entry from user/include/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>