vfs: escape hash as well

When a filesystem is mounted with a name that starts with a #:

 # mount '#name' /mnt/bad -t tmpfs

this will cause the entry to look like this (leading space added so
that git does not strip it out):

 #name /mnt/bad tmpfs rw,seclabel,relatime,inode64 0 0

This breaks getmntent and any code that aims to parse fstab as well as
/proc/mounts with the same logic since they need to strip leading spaces
or skip over comment lines, due to which they report incorrect output or
skip over the line respectively.

Solve this by translating the hash character into its octal encoding
equivalent so that applications can decode the name correctly.
Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc_namespace.c

@@ -86,7 +86,7 @@ static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
 
 static inline void mangle(struct seq_file *m, const char *s)
 {
-	seq_escape(m, s, " \t\n\\");
+	seq_escape(m, s, " \t\n\\#");
 }
 
 static void show_type(struct seq_file *m, struct super_block *sb)