proc/fd: In fdinfo seq_show don't use get_files_struct

When discussing[1] exec and posix file locks it was realized that none
of the callers of get_files_struct fundamentally needed to call
get_files_struct, and that by switching them to helper functions
instead it will both simplify their code and remove unnecessary
increments of files_struct.count.  Those unnecessary increments can
result in exec unnecessarily unsharing files_struct which breaking
posix locks, and it can result in fget_light having to fallback to
fget reducing system performance.

Instead hold task_lock for the duration that task->files needs to be
stable in seq_show.  The task_lock was already taken in
get_files_struct, and so skipping get_files_struct performs less work
overall, and avoids the problems with the files_struct reference
count.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-12-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-17-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

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