[JFFS2] Improve getdents vs. f_pos handling on NOR flash.

Commit a491486a started obliterating
dirents directly on the medium, when jffs2_can_mark_obsolete(). Removing
them immediately from the f->dents list, however, screws up handling of
f_pos within a directory -- because the offset is equivalent to the
number of entries through the list we are, and the existence of
deletion dirents served to provide 'placeholders' for unlinked
entries. Now, 'rm -r' doesn't even manage to unlink everything in the
directory.

Revert to keeping 'deletion' dirents in the list, at least in memory
even though we no longer write anything to the medium.

Spotted, debugged and mostly fixed by Joakim Tjernlund

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