file: Remove get_files_struct

file: Remove 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.

Now that get_files_struct has no more users and can not cause the
problems for posix file locking and fget_light remove get_files_struct
so that it does not gain any new users.

[1] https://lkml.kernel.org/r/20180915160423[email protected]
Suggested-by: Oleg Nesterov <[email protected]>
Acked-by: Christian Brauner <[email protected]>
v1: https://lkml.kernel.org/r/20200817220425[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Eric W. Biederman <[email protected]>