ashmem: Validate ashmem memory with fops pointer

Validate the ashmem memory entry against f_op pointer
rather then comparing its name with path of the dentry.

This is to avoid any invalid access to ashmem area in cases
where some one deliberately set the dentry name to /ashmem.

Change-Id: I74e50cd244f68cb13009cf2355e528485f4de34b
Signed-off-by: Sunil Khatri <sunilkh@codeaurora.org>