ceph: minor cleanup in ceph_fscrypt_decrypt_extents()

ceph: minor cleanup in ceph_fscrypt_decrypt_extents()

The Coverity Scan service has reported a potential issue
in ceph_fscrypt_decrypt_extents() method [1]. The function
ceph_fscrypt_decrypt_page() can return the negative value as
an error code. Logic of ceph_fscrypt_decrypt_extents()
process this case in correct way. However, it makes sense
to make the minor cleanup of the function logic.

This patch adds several unlikely macros to conditions checks
and it reworks fret variable check by adding else statement
to the condition check.

[1] https://scan5.scan.coverity.com/#/project-view/64304/10063?selectedIssue=1662519

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
cc: Alex Markuze <amarkuze@redhat.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Ceph Development <ceph-devel@vger.kernel.org>