ntfs3: avoid -Wmaybe-uninitialized warning

ntfs3: avoid -Wmaybe-uninitialized warning

This warning shows up with gcc-10 now:

In file included from fs/ntfs3/index.c:15:
fs/ntfs3/index.c: In function 'indx_add_allocate':
fs/ntfs3/ntfs_fs.h:463:9: error: 'bmp_size' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  463 |  return attr_set_size_ex(ni, type, name, name_len, run, new_size,
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  464 |     new_valid, keep_prealloc, NULL, false);
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
fs/ntfs3/index.c:1498:6: note: 'bmp_size' was declared here
 1498 |  u64 bmp_size, bmp_size_v;
      |      ^~~~~~~~

The warning does look correct, as the 'out2' label can be reached
without initializing bmp_size and bmp_size_v. Initialize these at
the same place as bmp.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>