mm: fadvise: move 'endbyte' calculations to helper function

mm: fadvise: move 'endbyte' calculations to helper function

Patch series "mm: shmem: support POSIX_FADV_[WILL|DONT]NEED for shmem
files", v7.

This patchset aims to implement POSIX_FADV_WILLNEED and
POSIX_FADV_DONTNEED advices to shmem files which can be helpful for the
drivers who may want to manage the pages of shmem files on their own,
like, that are created through shmem_file_setup[_with_mnt]().

This patch (of 2):

Move the 'endbyte' calculations that determines last byte that fadvise can
to a helper function.  This is a preparatory change made for
shmem_fadvise() functionality in the next patch.  No functional changes in
this patch.

Link: https://lkml.kernel.org/r/cover.1676378702.git.quic_charante@quicinc.com
Link: https://lkml.kernel.org/r/22de7e716051abbafc01fab9f479f4d5b03745ca.1676378702.git.quic_charante@quicinc.com
Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mark Hemment <markhemm@googlemail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>