Commit 1aa7b7b2 ("fs: document known deadlocks") describes a deadlock
scenario where inheriting file descriptors triggers a spurious close
during subprocess setup, leading to deadlock.

This exact scenario actually happens when setting up the FUSE mount
using fusermount: the fusermount process inherits one half of a socket
pair, which is used to pass back the opened /dev/fuse file.  After the
mount is successful, we open a file in the FUSE mount for the poll
hack. This means that in parallel scenarios, we may use fd 3 as the
poll hack, while also calling fusermount with inherited file
descriptors.

Solve this by grabbing fd 3 during initialization.

This is not completely foolproof, as FD 0, 1 and 2 could be closed
(and then reused) after initialization finished, but this should be
uncommon as 0, 1 and 2 are standard input/output/error.

Doing it during the init phase means that we prevent deadlock for all
users that inherit single file descriptors.

Change-Id: If5ac7c941f0ee2e13ca657c31d056a676eed3fde