fs: crypto: Fix memory leak while allocating blk crypto key

In case of legaacy data encryption mode, block crypto key allocated
for file encryption was not freed when inode is evicted or destroyed
as crypto data do not own the key. So free the blk crpto key
explicitly in case legacy data encryption mode is used.

Change-Id: Ia860edc7c67516bf8fa47b6e3241ce068a09492b
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>