eCryptfs: Infinite loop due to overflow in ecryptfs_write()

eCryptfs: Infinite loop due to overflow in ecryptfs_write()

ecryptfs_write() can enter an infinite loop when truncating a file to a
size larger than 4G. This only happens on architectures where size_t is
represented by 32 bits.

This was caused by a size_t overflow due to it incorrectly being used to
store the result of a calculation which uses potentially large values of
type loff_t.

[[email protected]: rewrite subject and commit message]
Signed-off-by: Li Wang <[email protected]>
Signed-off-by: Yunchuan Wen <[email protected]>
Reviewed-by: Cong Wang <[email protected]>
Cc: <[email protected]>
Signed-off-by: Tyler Hicks <[email protected]>