The macro get_unused_fd() is used to allocate a file descriptor with
default flags.  Those default flags (0) can be "unsafe": O_CLOEXEC must
be used by default to not leak file descriptor across exec().

Replace calls to get_unused_fd() in uverbs with calls to
get_unused_fd_flags(O_CLOEXEC).  Inheriting uverbs fds across exec()
cannot be used to do anything useful.

Based on a patch/suggestion from Yann Droneaud <ydroneaud@opteya.com>.
Signed-off-by: Roland Dreier <roland@purestorage.com>