memstick: avert possible race condition between idr_pre_get and idr_get_new

Implement the usual pattern around idr_pre_get() and idr_get_new() to
handlethe situation where another thread concurrently steals this thread's
idr_pre_get() preallocation.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>