fs: btrfs: fix reading when length specified

The btrfs read function limits the read length to ensure that it
and the read offset do not together exceed the size of the file.
However, this size was only being queried if the read length was
passed a value of zero (meaning "whole file"), and the size is
defaulted to 0 otherwise. This means the clamp will just zero out
the length if one is specified, preventing reading of the file.

Fix this by checking the file size unconditionally, and unifying
the default length and clamping logic as a single range check instead.

This bug was discovered when trying to boot Linux with initrd= via
'bootefi' from a btrfs partition. The EFI stub entered an infinite
loop of zero-length reads while trying to read the initrd, and the
boot process stalled indefinitely.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>