autofs: fix missing fput for FSCONFIG_SET_FD

If you pass an fd using FSCONFIG_SET_FD, autofs_parse_fd() "steals" the
param->file and so the fs_context infrastructure will not do fput() for
us.

Fixes: e6ec453b ("autofs: convert autofs to use the new mount api")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20240731-fsconfig-fsparam_fd-fixes-v2-1-e7c472224417@cyphar.com


Signed-off-by: Christian Brauner <brauner@kernel.org>