ANDROID: mm: shmem: use reclaim_pages() to recalim pages from a list

Static code analysis tool reported NULL pointer access in
shrink_page_list() as the commit 26aa2d19 ("mm/migrate: demote
pages during reclaim") expects valid pgdat.

There is already an existing api, reclaim_pages, that tries to reclaim
pages from the list. use it instead of creating custom function.

Bug: 201263305
Fixes: 96f80f62 ("ANDROID: mm: add reclaim_shmem_address_space() for faster reclaims")
Change-Id: Iaa11feac94c9e8338324ace0276c49d6a0adeb0c
Signed-off-by: Charan Teja Reddy <quic_charante@quicinc.com>