proc doesn't use "associate pointer with id" feature of IDR, so switch
to IDA.

NOTE, NOTE, NOTE:
	Do not apply if release_inode_number() still mantions MAX_ID_MASK!
Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>