xfs: fix string handling in label get/set functions

xfs: fix string handling in label get/set functions

[sandeen: fix subject, avoid copy-out of uninit data in getlabel]

gcc-8 reports two warnings for the newly added getlabel/setlabel code:

fs/xfs/xfs_ioctl.c: In function 'xfs_ioc_getlabel':
fs/xfs/xfs_ioctl.c:1822:38: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
  strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));
                                      ^
In function 'strncpy',
    inlined from 'xfs_ioc_setlabel' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1863:2,
    inlined from 'xfs_file_ioctl' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1918:10:
include/linux/string.h:254:9: error: '__builtin_strncpy' output may be truncated copying 12 bytes from a string of length 12 [-Werror=stringop-truncation]
  return __builtin_strncpy(p, q, size);

In both cases, part of the problem is that one of the strncpy()
arguments is a fixed-length character array with zero-padding rather
than a zero-terminated string. In the first one case, we also get an
odd warning about sizeof-pointer-memaccess, which doesn't seem right
(the sizeof is for an array that happens to be the same as the second
strncpy argument).

To work around the bogus warning, I use a plain 'XFSLABEL_MAX' for
the strncpy() length when copying the label in getlabel. For setlabel(),
using memcpy() with the correct length that is already known avoids
the second warning and is slightly simpler.

In a related issue, it appears that we accidentally skip the trailing
\0 when copying a 12-character label back to user space in getlabel().
Using the correct sizeof() argument here copies the extra character.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602
Fixes: f7664b31975b ("xfs: implement online get/set fs label")
Cc: Eric Sandeen <sandeen@redhat.com>
Cc: Martin Sebor <msebor@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>