mm/filemap: use filemap_read_page in filemap_fault

After splitting generic_file_buffered_read() into smaller parts, it turns
out we can reuse one of the parts in filemap_fault().  This fixes an
oversight -- waiting for the I/O to complete is now interruptible by a
fatal signal.  And it saves us a few bytes of text in an unlikely path.

  $ ./scripts/bloat-o-meter before.o after.o
  add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-207 (-207)
  Function                                     old     new   delta
  filemap_fault                               2187    1980    -207
  Total: Before=37491, After=37284, chg -0.55%

Link: https://lkml.kernel.org/r/20210226140011.2883498-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>