Avoid nested function declarations in helper functions

Avoid nested function declarations in helper functions

libfuse-3.17 introduced several functions that should only be called via
inlined helper functions, never directly. To enforce this, these functions
were declared within the inlined functions. However, this triggers the
compiler warning "-Werror=nested-externs".

While this warning is valid, the nested declarations were intentional to
prevent direct usage of these functions. Rather than suppressing the
warning with pragmas, move these function declarations outside the helper
functions while maintaining the intended access restrictions through other
means.

Closes: https://github.com/libfuse/libfuse/issues/1134
Signed-off-by: Bernd Schubert <bschubert@ddn.com>