bcachefs: made changes to support clang, fixed a couple bugs

bcachefs: made changes to support clang, fixed a couple bugs

fs/bcachefs/bset.c              edited prefetch macro to add clang support
fs/bcachefs/btree_iter.c        bugfix: initialize iter->real_pos in bch2_btree_iter_init for later use
fs/bcachefs/io.c                bugfix: eliminated undefined behavior (negative bitshift)
fs/bcachefs/buckets.c           bugfix: invert sign to handle 64bit abs()

Signed-off-by: Brett Holman <bpholman5@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>