[JFFS2] Clean up jffs2_alloc_inode() and jffs2_i_init_once()

Ditch a couple of pointless casts from void *, and use the normal
variable name 'f' for jffs2_inode_info pointers -- especially since
it actually shows up in lockdep reports.

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