eventfd: convert to use anon_inode_getfd()

Nothing actually calls eventfd_file_create() besides the eventfd2()
system call itself.  So simplify things by folding it into the system
call and using anon_inode_getfd() instead of anon_inode_getfile().  This
removes over 40 lines with no change in functionality.

(eventfd_file_create() was apparently added years ago for KVM irqfd's,
but was never used.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>