fs/ntfs3: Remove redundant assignment to variable vcn

Variable vcn is being assigned a value that is never read, it is
being re-assigned again in the initialization of a for-loop.  The
assignment is redundant and can be removed.

Cleans up clang scan build warning:
fs/ntfs3/attrib.c:1176:7: warning: Value stored to 'vcn' during its
initialization is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Kari Argillander <kari.argillander@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>