vfs: add the sb_start_intwrite_trylock() helper

Needed by ext4 to test frozen fs before updating s_last_mounted.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

include/linux/fs.h

@@ -1597,6 +1597,11 @@ static inline void sb_start_intwrite(struct super_block *sb)
 	__sb_start_write(sb, SB_FREEZE_FS, true);
 }
 
+static inline int sb_start_intwrite_trylock(struct super_block *sb)
+{
+	return __sb_start_write(sb, SB_FREEZE_FS, false);
+}
+
 
 extern bool inode_owner_or_capable(const struct inode *inode);