[JFFS2] Fix cross-endian build.

When compiling a LE-capable JFFS2 on PowerPC, wbuf.c fails to compile:

fs/jffs2/wbuf.c:973: error: braced-group within expression allowed only inside a function
fs/jffs2/wbuf.c:973: error: initializer element is not constant
fs/jffs2/wbuf.c:973: error: (near initialization for ‘oob_cleanmarker.magic’)
fs/jffs2/wbuf.c:974: error: braced-group within expression allowed only inside a function
fs/jffs2/wbuf.c:974: error: initializer element is not constant
fs/jffs2/wbuf.c:974: error: (near initialization for ‘oob_cleanmarker.nodetype’)
fs/jffs2/wbuf.c:975: error: braced-group within expression allowed only inside a function
fs/jffs2/wbuf.c:976: error: initializer element is not constant
fs/jffs2/wbuf.c:976: error: (near initialization for ‘oob_cleanmarker.totlen’)

Provide constant_cpu_to_je{16,32} functions, and use them for initialising the
offending structure.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>