BtrfsFileStoreBackend.cc: fix ::unlinkat() result handling

Don't check for 'fd' but for the return value of the ::unlinkat() call.

Fix for:
[src/os/BtrfsFileStoreBackend.cc:72] -> [src/os/BtrfsFileStoreBackend.cc:74]:
 (warning) Opposite conditions in nested 'if' blocks lead to a dead code block.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>