vfs, fdtable: Add fget_task helper

This introduces a function which can be used to fetch a file, given an
arbitrary task. As long as the user holds a reference (refcnt) to the
task_struct it is safe to call, and will either return NULL on failure,
or a pointer to the file, with a refcnt.

This patch is based on Oleg Nesterov's (cf. [1]) patch from September
2018.

[1]: Link: https://lore.kernel.org/r/[email protected]

Signed-off-by: Sargun Dhillon <[email protected]>
Suggested-by: Oleg Nesterov <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Reviewed-by: Arnd Bergmann <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

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