[PATCH v3] fs: namespace: suppress 'may be used uninitialized' warnings

From: Tim Gardner <tim.gardner@canonical.com>

The gcc version 4.9.1 compiler complains Even though it isn't possible for
these variables to not get initialized before they are used.

fs/namespace.c: In function ‘SyS_mount’:
fs/namespace.c:2720:8: warning: ‘kernel_dev’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  ret = do_mount(kernel_dev, kernel_dir->name, kernel_type, flags,
        ^
fs/namespace.c:2699:8: note: ‘kernel_dev’ was declared here
  char *kernel_dev;
        ^
fs/namespace.c:2720:8: warning: ‘kernel_type’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  ret = do_mount(kernel_dev, kernel_dir->name, kernel_type, flags,
        ^
fs/namespace.c:2697:8: note: ‘kernel_type’ was declared here
  char *kernel_type;
        ^

Fix the warnings by simplifying copy_mount_string() as suggested by Al Viro.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
---
V1 - use of the uninitialized_var() macro rejected.
V2 - assign automatic variables an initial value.
V3 - minor surgery to simplify copy_mount_string(), thereby removing all doubt as to whether
     these variables are uninitialized.
Fix breakage of compilation by reverting (not needing now - so reverting)

Revert "[PATCH] btrfs: Make btrfs handle security mount options internally to avoid losing security label."

This reverts commit 913b373cab31cc5cbe265a801e852765cc3adab6.
Merge branch 'btrfs-integration_13.09.2014+fixes_18.09.2014_v2' into linux-3.16.3-plus-v2_07_WIP
Merge branch 'linux-3.16.0-misc-fixes' into linux-3.16.3-plus-v2_07_WIP