pfk: fixed bug that caused eCryptfs ignore some of its files

Our logic that distinguished eCryptfs files from non eCryptfs ones
checked bi_vcnt field in bio to make sure it has valid bio vecs.
Apparently this field is not updated in some cases by block layer
and in general can't be trusted outside of code that own bio.
There is another field that need to be used for this purpose - bi_size

Change-Id: Ibf225543618373699a0d5adc3295daf9ded6169a
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>