MINOR: fd: make fd_clr_running() return the remaining running mask

MINOR: fd: make fd_clr_running() return the remaining running mask

We'll need to know that a thread is the last one to use an fd, so let's
make fd_clr_running() return the remaining bits after removal. Note that
in practice we're only interested in knowing if it's zero but the compiler
doesn't make use of the clags after the AND and emits a CMPXCHG anyway :-/

This will need to be backported to 2.2 to fix an issue.

(cherry picked from commit 6e8e10b4159f4a31e6aaa535e38dc035f8711787)
Signed-off-by: Willy Tarreau <w@1wt.eu>