inotify: convert to idr_alloc()

inotify: convert to idr_alloc()

Convert to the much saner new idr interface.

Note that the adhoc cyclic id allocation is buggy.  If wraparound
happens, the previous code with idr_get_new_above() may segfault and
the converted code will trigger WARN and return -EINVAL.  Even if it's
fixed to wrap to zero, the code will be prone to unnecessary -ENOSPC
failures after the first wraparound.  We probably need to implement
proper cyclic support in idr.

Signed-off-by: Tejun Heo <[email protected]>
Cc: John McCutchan <[email protected]>
Cc: Robert Love <[email protected]>
Cc: Eric Paris <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>