ext2fs.h: move ext2fs_init_csum_seed() outside of EXT2_CUSTOM_MEMORY_ROUTINES

ext2fs.h: move ext2fs_init_csum_seed() outside of EXT2_CUSTOM_MEMORY_ROUTINES

The function ext2fs_init_csum_seed() has nothing to do with the
ext2fs_get_mem()/ext2fs_get_memzero()/ext2fs_get_array()/ext2fs_get_arrayzero()
functions.  (This define is there so that on platforms where we need
to use the standard C functions, they can be replaced --- this is
primarily needed when trying to compile libext2fs for strange,
non-quite-standards-compliant platforms, such as Windows.)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>