vfs: replace WARN(down_read_trylock, ...) abuse with proper asserts

Note the macro used here works regardless of LOCKDEP.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240602123720.775702-1-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

fs/dcache.c

@@ -1548,7 +1548,7 @@ void shrink_dcache_for_umount(struct super_block *sb)
 {
 	struct dentry *dentry;
 
-	WARN(down_read_trylock(&sb->s_umount), "s_umount should've been locked");
+	rwsem_assert_held_write(&sb->s_umount);
 
 	dentry = sb->s_root;
 	sb->s_root = NULL;

fs/quota/dquot.c

@@ -2246,9 +2246,7 @@ int dquot_disable(struct super_block *sb, int type, unsigned int flags)
 	int cnt;
 	struct quota_info *dqopt = sb_dqopt(sb);
 
-	/* s_umount should be held in exclusive mode */
-	if (WARN_ON_ONCE(down_read_trylock(&sb->s_umount)))
-		up_read(&sb->s_umount);
+	rwsem_assert_held_write(&sb->s_umount);
 
 	/* Cannot turn off usage accounting without turning off limits, or
 	 * suspend quotas and simultaneously turn quotas off. */
@@ -2510,9 +2508,7 @@ int dquot_resume(struct super_block *sb, int type)
 	int ret = 0, cnt;
 	unsigned int flags;
 
-	/* s_umount should be held in exclusive mode */
-	if (WARN_ON_ONCE(down_read_trylock(&sb->s_umount)))
-		up_read(&sb->s_umount);
+	rwsem_assert_held_write(&sb->s_umount);
 
 	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
 		if (type != -1 && cnt != type)