IB/uverbs: Use anon_inodes instead of private infinibandeventfs

The anon_inodes interface has been split to allow creating a bare
(non-installed) file pointer and also extended to allow specifying
O_RDONLY in the flags.  This makes it a suitable replacement for the
private "infinibandeventfs" pseudo-filesystem used by uverbs, and this
replacement saves a small chunk of boilerplate code.

Signed-off-by: Roland Dreier <rolandd@cisco.com>