Address -Wframe-larger-than warnings in fs/mpage.c

[GustavoARSilva]

Found after building next-20230620 (with GCC-13) sh with shx3_defconfig:

fs/mpage.c: In function '__mpage_writepage':
fs/mpage.c:657:1: warning: the frame size of 1144 bytes is larger than 1024 bytes [-Wframe-larger-than=]
  657 | }
      | ^
fs/mpage.c: In function 'do_mpage_readpage':
fs/mpage.c:329:1: warning: the frame size of 1092 bytes is larger than 1024 bytes [-Wframe-larger-than=]
  329 | }
      | ^