nilfs2: fix deadlock warnings caused by lock dependency in init_nilfs()

After commit c0e473a0 ("block: fix race between set_blocksize and read
paths") was merged, set_blocksize() called by sb_set_blocksize() now locks
the inode of the backing device file.  As a result of this change, syzbot
started reporting deadlock warnings due to a circular dependency involving
the semaphore "ns_sem" of the nilfs object, the inode lock of the backing
device file, and the locks that this inode lock is transitively dependent
on.

This is caused by a new lock dependency added by the above change, since
init_nilfs() calls sb_set_blocksize() in the lock section of "ns_sem". 
However, these warnings are false positives because init_nilfs() is called
in the early stage of the mount operation and the filesystem has not yet
started.

The reason why "ns_sem" is locked in init_nilfs() was to avoid a race
condition in nilfs_fill_super() caused by sharing a nilfs object among
multiple filesystem instances (super block structures) in the early
implementation.  However, nilfs objects and super block structures have
long ago become one-to-one, and there is no longer any need to use the
semaphore there.

So, fix this issue by removing the use of the semaphore "ns_sem" in
init_nilfs().

Link: https://lkml.kernel.org/r/20250503053327.12294-1-konishi.ryusuke@gmail.com


Fixes: c0e473a0 ("block: fix race between set_blocksize and read paths")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by:  <syzbot+00f7f5b884b117ee6773@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=00f7f5b884b117ee6773


Tested-by:  <syzbot+00f7f5b884b117ee6773@syzkaller.appspotmail.com>
Reported-by:  <syzbot+f30591e72bfc24d4715b@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=f30591e72bfc24d4715b


Tested-by:  <syzbot+f30591e72bfc24d4715b@syzkaller.appspotmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>