eCryptfs: Make extent and scatterlist crypt function parameters similar

The 'dest' abbreviation is only used in crypt_scatterlist(), while all
other functions in crypto.c use 'dst' so dest_sg should be renamed to
dst_sg.

The crypt_stat parameter is typically the first parameter in internal
eCryptfs functions so crypt_stat and dst_page should be swapped in
crypt_extent().

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>