fs: remove fget_many and fput_many interface

These two interface were added in 091141a4 commit,
but now there is no place to call them.

The only user of fput/fget_many() was removed in commit
62906e89 ("io_uring: remove file batch-get optimisation").

A user of get_file_rcu_many() were removed in commit
f0735310

 ("init: add an init_dup helper").

And replace atomic_long_sub/add to atomic_long_dec/inc
can improve performance.

Here are the test results of unixbench:

Cmd: ./Run -c 64 context1

Without patch:
System Benchmarks Partial Index              BASELINE       RESULT    INDEX
Pipe-based Context Switching                   4000.0    2798407.0   6996.0
                                                                   ========
System Benchmarks Index Score (Partial Only)                         6996.0

With patch:
System Benchmarks Partial Index              BASELINE       RESULT    INDEX
Pipe-based Context Switching                   4000.0    3486268.8   8715.7
                                                                   ========
System Benchmarks Index Score (Partial Only)                         8715.7

Signed-off-by: Gou Hao <gouhao@uniontech.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>