file: In f_dupfd read RLIMIT_NOFILE once.

Simplify the code, and remove the chance of races by reading
RLIMIT_NOFILE only once in f_dupfd.

Pass the read value of RLIMIT_NOFILE into alloc_fd which is the other
location the rlimit was read in f_dupfd.  As f_dupfd is the only
caller of alloc_fd this changing alloc_fd is trivially safe.

Further this causes alloc_fd to take all of the same arguments as
__alloc_fd except for the files_struct argument.
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-15-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-19-ebiederm@xmission.comSigned-off-by: Eric W. Biederman <ebiederm@xmission.com>

fs/file.c

@@ -538,9 +538,9 @@ int __alloc_fd(struct files_struct *files,
 	return error;
 }
 
-static int alloc_fd(unsigned start, unsigned flags)
+static int alloc_fd(unsigned start, unsigned end, unsigned flags)
 {
-	return __alloc_fd(current->files, start, rlimit(RLIMIT_NOFILE), flags);
+	return __alloc_fd(current->files, start, end, flags);
 }
 
 int __get_unused_fd_flags(unsigned flags, unsigned long nofile)
@@ -1175,10 +1175,11 @@ SYSCALL_DEFINE1(dup, unsigned int, fildes)
 
 int f_dupfd(unsigned int from, struct file *file, unsigned flags)
 {
+	unsigned long nofile = rlimit(RLIMIT_NOFILE);
 	int err;
-	if (from >= rlimit(RLIMIT_NOFILE))
+	if (from >= nofile)
 		return -EINVAL;
-	err = alloc_fd(from, flags);
+	err = alloc_fd(from, nofile, flags);
 	if (err >= 0) {
 		get_file(file);
 		fd_install(err, file);