reiser4: fix problem of non-deletable directories

Problem:
parse_cut() calculates cut mode incorrectly when the range contains
objects with non-unique keys, e.g. directory items.

Scenario of corruption.
A node contains only one cde-item with 44 (#0-#43) units, and the units
#42,#43 have identical keys (because of hash collision). We shift units
#0-#42 to left. In this case parse_cut() jumps to the branch "removed
completely". As the result the whole item is removed, while only units
#0-#42 are copied to the left neighbor. unlink() can not find directory
entry which corresponds to unexpectedly killed unit #43. rmdir() doesn't
decrement size of the directory in the error path. As the result, the
directory becomes non-deletable.

Fixup:
For ranges with non-unique keys use a special version of parse_cut(),
which doesn't calculate cut mode by keys.

Signed-off-by: Edward Shishkin <edward.shishkin@gmail.com>