ANDROID: sdcardfs: Propagate dentry down to inode_change_ok()

Since commit 31051c85 ("fs: Give dentry to inode_change_ok()
instead of inode"), to avoid clearing of capabilities or security
related extended attributes too early, inode_change_ok() will
need to take dentry instead of inode. Propagate it down to
sdcardfs_setattr() and also rename it to setattr_prepare().

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>