ANDROID: sdcardfs: Don't d_drop in d_revalidate

After d_revalidate returns 0, the vfs will call
d_invalidate, which will call d_drop itself, along
with other cleanup.

Bug: 78262592
Change-Id: Idbb30e008c05d62edf2217679cb6a5517d8d1a2c
Signed-off-by: Daniel Rosenberg <drosen@google.com>