bcachefs: clean up some dead fallocate code

The have_reservation local variable in bch2_extent_fallocate() is
initialized to false and set to true further down in the function.
Between this two points, one branch of code checks for negative
value and one for positive, and nothing ever checks the variable
after it is set to true. Clean up some of the unnecessary logic and
code.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>