fs/ntfs3: Fix fall-through warnings for Clang

Fix the following fallthrough warnings:

fs/ntfs3/inode.c:1792:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/ntfs3/index.c:178:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

This helps with the ongoing efforts to globally enable
-Wimplicit-fallthrough for Clang.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>