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.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-13-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-24-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

fs/file.c

@@ -411,19 +411,6 @@ static struct fdtable *close_files(struct files_struct * files)
 	return fdt;
 }
 
-struct files_struct *get_files_struct(struct task_struct *task)
-{
-	struct files_struct *files;
-
-	task_lock(task);
-	files = task->files;
-	if (files)
-		atomic_inc(&files->count);
-	task_unlock(task);
-
-	return files;
-}
-
 void put_files_struct(struct files_struct *files)
 {
 	if (atomic_dec_and_test(&files->count)) {

include/linux/fdtable.h

@@ -115,7 +115,6 @@ struct file *task_lookup_next_fd_rcu(struct task_struct *task, unsigned int *fd)
 
 struct task_struct;
 
-struct files_struct *get_files_struct(struct task_struct *);
 void put_files_struct(struct files_struct *fs);
 int unshare_files(void);
 struct files_struct *dup_fd(struct files_struct *, unsigned, int *) __latent_entropy;